Lithium-ion battery

Research on rechargeable Li-ion batteries dates to the 1960s; one of the earliest examples is a CuF
₂/Li battery developed by NASA in 1965. The breakthrough that produced the earliest form of the modern Li-ion battery was made by British chemist M. Stanley Whittingham in 1974, who first used titanium disulfide (TiS
₂) as a cathode material, which has a layered structure that can take in lithium ions without significant changes to its crystal structure. Exxon tried to commercialize this battery in the late 1970s, but found the synthesis expensive and complex, as TiS
₂ is sensitive to moisture and releases toxic H
₂S gas on contact with water. More prohibitively, the batteries were also prone to spontaneously catch fire due to the presence of metallic lithium in the cells. For this, and other reasons, Exxon discontinued the development of Whittingham's lithium-titanium disulfide battery.^[25]

In 1980, working in separate groups Ned A. Godshall et al.,^[26][27][28] and, shortly thereafter, Koichi Mizushima and John B. Goodenough, after testing a range of alternative materials, replaced TiS
₂ with lithium cobalt oxide (LiCoO
₂, or LCO), which has a similar layered structure but offers a higher voltage and is much more stable in air. This material would later be used in the first commercial Li-ion battery, although it did not, on its own, resolve the persistent issue of flammability.^[25]

These early attempts to develop rechargeable Li-ion batteries used lithium metal anodes, which were ultimately abandoned due to safety concerns, as lithium metal is unstable and prone to dendrite formation, which can cause short-circuiting. The eventual solution was to use an intercalation anode, similar to that used for the cathode, which prevents the formation of lithium metal during battery charging. The first to demonstrate lithium ion reversible intercalation into graphite anodes was Jürgen Otto Besenhard in 1974.^[29][30] Besenhard used organic solvents such as carbonates, however these solvents decomposed rapidly providing short battery cycle life. Later, in 1980, Rachid Yazami used a solid organic electrolyte, polyethylene oxide, which was more stable.^[31][32]

In 1985, Akira Yoshino at Asahi Kasei Corporation discovered that petroleum coke, a less graphitized form of carbon, can reversibly intercalate Li-ions at a low potential of ~0.5 V relative to Li+ /Li without structural degradation.^[33] Its structural stability originates from its amorphous carbon regions, which serving as covalent joints to pin the layers together. Although it has a lower capacity compared to graphite (~Li0.5C6, 186 mAh g–1), it became the first commercial intercalation anode for Li-ion batteries owing to its cycling stability. In 1987, Yoshino patented what would become the first commercial lithium-ion battery using this anode. He used Goodenough's previously reported LiCoO₂ as the cathode and a carbonate ester-based electrolyte. The battery was assembled in the discharged state, which made it safer and cheaper to manufacture. In 1991, using Yoshino's design, Sony began producing and selling the world's first rechargeable lithium-ion batteries. The following year, a joint venture between Toshiba and Asashi Kasei Co. also released a lithium-ion battery.^[25]

Significant improvements in energy density were achieved in the 1990s by replacing Yoshino's soft carbon anode first with hard carbon and later with graphite. In 1990, Jeff Dahn and two colleagues at Dalhousie University (Canada) reported reversible intercalation of lithium ions into graphite in the presence of ethylene carbonate solvent (which is solid at room temperature and is mixed with other solvents to make a liquid). This represented the final innovation of the era that created the basic design of the modern lithium-ion battery.^[34]

In 2010, global lithium-ion battery production capacity was 20 gigawatt-hours.^[35] By 2016, it was 28 GWh, with 16.4 GWh in China.^[36] Global production capacity was 767 GWh in 2020, with China accounting for 75%.^[37] Production in 2021 is estimated by various sources to be between 200 and 600 GWh, and predictions for 2023 range from 400 to 1,100 GWh.^[38]

In 2012, John B. Goodenough, Rachid Yazami and Akira Yoshino received the 2012 IEEE Medal for Environmental and Safety Technologies for developing the lithium-ion battery; Goodenough, Whittingham, and Yoshino were awarded the 2019 Nobel Prize in Chemistry "for the development of lithium-ion batteries".^[39] Jeff Dahn received the ECS Battery Division Technology Award (2011) and the Yeager award from the International Battery Materials Association (2016).

In April 2023, CATL announced that it would begin scaled-up production of its semi-solid condensed matter battery that produces a then record 500 Wh/kg. They use electrodes made from a gelled material, requiring fewer binding agents. This in turn shortens the manufacturing cycle. One potential application is in battery-powered airplanes.^[40][41][42] Another new development of lithium-ion batteries are flow batteries with redox-targeted solids, that use no binders or electron-conducting additives, and allow for completely independent scaling of energy and power.^[43]

Generally, the negative electrode of a conventional lithium-ion cell is graphite made from carbon. The positive electrode is typically a metal oxide or phosphate. The electrolyte is a lithium salt in an organic solvent.^[44] The negative electrode (which is the anode when the cell is discharging) and the positive electrode (which is the cathode when discharging) are prevented from shorting by a separator.^[45] The electrodes are connected to the powered circuit through two pieces of metal called current collectors.^[46]

The negative and positive electrodes swap their electrochemical roles (anode and cathode) when the cell is charged. Despite this, in discussions of battery design the negative electrode of a rechargeable cell is often just called "the anode" and the positive electrode "the cathode".

In its fully lithiated state of LiC₆, graphite correlates to a theoretical capacity of 1339 coulombs per gram (372 mAh/g).^[47] The positive electrode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as lithium manganese oxide).^[48] More experimental materials include graphene-containing electrodes, although these remain far from commercially viable due to their high cost.^[49]

Lithium reacts vigorously with water to form lithium hydroxide (LiOH) and hydrogen gas. Thus, a non-aqueous electrolyte is typically used, and a sealed container rigidly excludes moisture from the battery pack. The non-aqueous electrolyte is typically a mixture of organic carbonates such as ethylene carbonate and propylene carbonate containing complexes of lithium ions.^[50] Ethylene carbonate is essential for making solid electrolyte interphase on the carbon anode,^[51] but since it is solid at room temperature, a liquid solvent (such as propylene carbonate or diethyl carbonate) is added.

The electrolyte salt is almost always^{[citation needed]} lithium hexafluorophosphate (LiPF
₆), which combines good ionic conductivity with chemical and electrochemical stability. The hexafluorophosphate anion is essential for passivating the aluminium current collector used for the positive electrode. A titanium tab is ultrasonically welded to the aluminium current collector. Other salts like lithium perchlorate (LiClO
₄), lithium tetrafluoroborate (LiBF
₄), and lithium bis(trifluoromethanesulfonyl)imide (LiC
₂F
₆NO
₄S
₂) are frequently used in research in tab-less coin cells, but are not usable in larger format cells,^[52] often because they are not compatible with the aluminium current collector. Copper (with a spot-welded nickel tab) is used as the current collector at the negative electrode.

Current collector design and surface treatments may take various forms: foil, mesh, foam (dealloyed), etched (wholly or selectively), and coated (with various materials) to improve electrical characteristics.^[46]

Depending on materials choices, the voltage, energy density, life, and safety of a lithium-ion cell can change dramatically. Current effort has been exploring the use of novel architectures using nanotechnology to improve performance. Areas of interest include nano-scale electrode materials and alternative electrode structures.^[53]

The reactants in the electrochemical reactions in a lithium-ion cell are the materials of the electrodes, both of which are compounds containing lithium atoms. Although many thousands of different materials have been investigated for use in lithium-ion batteries, only a very small number are commercially usable. All commercial Li-ion cells use intercalation compounds as active materials.^[54] The negative electrode is usually graphite, although silicon is often mixed in to increase the capacity. The electrolyte is usually lithium hexafluorophosphate, dissolved in a mixture of organic carbonates. A number of different materials are used for the positive electrode, such as LiCoO₂, LiFePO₄, and lithium nickel manganese cobalt oxides.

During cell discharge the negative electrode is the anode and the positive electrode the cathode: electrons flow from the anode to the cathode through the external circuit. An oxidation half-reaction at the anode produces positively charged lithium ions and negatively charged electrons. The oxidation half-reaction may also produce uncharged material that remains at the anode. Lithium ions move through the electrolyte; electrons move through the external circuit toward the cathode where they recombine with the cathode material in a reduction half-reaction. The electrolyte provides a conductive medium for lithium ions but does not partake in the electrochemical reaction. The reactions during discharge lower the chemical potential of the cell, so discharging transfers energy from the cell to wherever the electric current dissipates its energy, mostly in the external circuit.

During charging these reactions and transports go in the opposite direction: electrons move from the positive electrode to the negative electrode through the external circuit. To charge the cell the external circuit has to provide electrical energy. This energy is then stored as chemical energy in the cell (with some loss, e. g., due to coulombic efficiency lower than 1).

Both electrodes allow lithium ions to move in and out of their structures with a process called insertion (intercalation) or extraction (deintercalation), respectively.

As the lithium ions "rock" back and forth between the two electrodes, these batteries are also known as "rocking-chair batteries" or "swing batteries" (a term given by some European industries).^[55][56]

The following equations exemplify the chemistry (left to right: discharging, right to left: charging).

The negative electrode half-reaction for the graphite is^[57][58]

${\displaystyle {\ce {LiC6 <=> C6 + Li+ + e^-}}}$ 

The positive electrode half-reaction in the lithium-doped cobalt oxide substrate is

${\displaystyle {\ce {CoO2 + Li+ + e- <=> LiCoO2}}}$ 

The full reaction being

${\displaystyle {\ce {LiC6 + CoO2 <=> C6 + LiCoO2}}}$ 

The overall reaction has its limits. Overdischarging supersaturates lithium cobalt oxide, leading to the production of lithium oxide,^[59] possibly by the following irreversible reaction:

${\displaystyle {\ce {Li+ + e^- + LiCoO2 -> Li2O + CoO}}}$ 

Overcharging up to 5.2 volts leads to the synthesis of cobalt (IV) oxide, as evidenced by x-ray diffraction:^[60]

${\displaystyle {\ce {LiCoO2 -> Li+ + CoO2 + e^-}}}$ 

The transition metal in the positive electrode, cobalt (Co), is reduced from Co⁴⁺
to Co³⁺
during discharge, and oxidized from Co³⁺
to Co⁴⁺
during charge.

The cell's energy is equal to the voltage times the charge. Each gram of lithium represents Faraday's constant/6.941, or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kilogram of lithium. This is a bit more than the heat of combustion of gasoline but does not consider the other materials that go into a lithium battery and that make lithium batteries many times heavier per unit of energy.

Note that the cell voltages involved in these reactions are larger than the potential at which an aqueous solutions would electrolyze.

During discharge, lithium ions (Li⁺
) carry the current within the battery cell from the negative to the positive electrode, through the non-aqueous electrolyte and separator diaphragm.^[61]

During charging, an external electrical power source applies an over-voltage (a voltage greater than the cell's own voltage) to the cell, forcing electrons to flow from the positive to the negative electrode. The lithium ions also migrate (through the electrolyte) from the positive to the negative electrode where they become embedded in the porous electrode material in a process known as intercalation.

Energy losses arising from electrical contact resistance at interfaces between electrode layers and at contacts with current collectors can be as high as 20% of the entire energy flow of batteries under typical operating conditions.^[62]

The charging procedures for single Li-ion cells, and complete Li-ion batteries, are slightly different:

• A single Li-ion cell is charged in two stages:^[63][64]

1. Constant current (CC)
2. Constant voltage (CV)

• A Li-ion battery (a set of Li-ion cells in series) is charged in three stages:

1. Constant current
2. Balance (only required when cell groups become unbalanced during use)
3. Constant voltage

During the constant current phase, the charger applies a constant current to the battery at a steadily increasing voltage, until the top-of-charge voltage limit per cell is reached.

During the balance phase, the charger/battery reduces the charging current (or cycles the charging on and off to reduce the average current) while the state of charge of individual cells is brought to the same level by a balancing circuit until the battery is balanced. Balancing typically occurs whenever one or more cells reach their top-of-charge voltage before the other(s), as it is generally inaccurate to do so at other stages of the charge cycle. This is most commonly done by passive balancing, which dissipates excess charge as heat via resistors connected momentarily across the cells to be balanced. Active balancing is less common, more expensive, but more efficient, returning excess energy to other cells (or the entire pack) via a DC-DC converter or other circuitry. Balancing most often occurs during the constant voltage stage of charging, switching between charge modes until complete. The pack is usually fully charged only when balancing is complete, as even a single cell group lower in charge than the rest will limit the entire battery's usable capacity to that of its own. Balancing can last hours or even days, depending on the magnitude of the imbalance in the battery.

During the constant voltage phase, the charger applies a voltage equal to the maximum cell voltage times the number of cells in series to the battery, as the current gradually declines towards 0, until the current is below a set threshold of about 3% of initial constant charge current.

Periodic topping charge about once per 500 hours. Top charging is recommended to be initiated when voltage goes below 4.05 V/cell. ^{[dubious – discuss]}

Failure to follow current and voltage limitations can result in an explosion.^[65][66]

Charging temperature limits for Li-ion are stricter than the operating limits. Lithium-ion chemistry performs well at elevated temperatures but prolonged exposure to heat reduces battery life. Li‑ion batteries offer good charging performance at cooler temperatures and may even allow "fast-charging" within a temperature range of 5 to 45 °C (41 to 113 °F).^{[67][better source needed]} Charging should be performed within this temperature range. At temperatures from 0 to 5 °C charging is possible, but the charge current should be reduced. During a low-temperature (under 0 °C) charge, the slight temperature rise above ambient due to the internal cell resistance is beneficial. High temperatures during charging may lead to battery degradation and charging at temperatures above 45 °C will degrade battery performance, whereas at lower temperatures the internal resistance of the battery may increase, resulting in slower charging and thus longer charging times.^{[67][better source needed]}

Batteries gradually self-discharge even if not connected and delivering current. Li-ion rechargeable batteries have a self-discharge rate typically stated by manufacturers to be 1.5–2% per month.^[68][69]

The rate increases with temperature and state of charge. A 2004 study found that for most cycling conditions self-discharge was primarily time-dependent; however, after several months of stand on open circuit or float charge, state-of-charge dependent losses became significant. The self-discharge rate did not increase monotonically with state-of-charge, but dropped somewhat at intermediate states of charge.^[70] Self-discharge rates may increase as batteries age.^[71] In 1999, self-discharge per month was measured at 8% at 21 °C, 15% at 40 °C, 31% at 60 °C.^[72] By 2007, monthly self-discharge rate was estimated at 2% to 3%, and 2^[6]–3% by 2016.^[73]

By comparison, the self-discharge rate for NiMH batteries dropped, as of 2017, from up to 30% per month for previously common cells^[74] to about 0.08–0.33% per month for low self-discharge NiMH batteries, and is about 10% per month in NiCd batteries.^{[citation needed]}

There are three classes of commercial cathode materials in lithium-ion batteries: (1) layered oxides, (2) spinel oxides and (3) oxoanion complexes. All of them were discovered by John Goodenough and his collaborators.^[75]

LiCoO₂ was used in the first commercial lithium-ion battery made by Sony in 1991. The layered oxides have a pseudo-tetrahedral structure comprising layers made of MO₆ octahedra separated by interlayer spaces that allow for two-dimensional lithium-ion diffusion.^{[citation needed]} The band structure of Li_xCoO₂ allows for true electronic (rather than polaronic) conductivity. However, due to an overlap between the Co⁴⁺ t_2g d-band with the O^2- 2p-band, the x must be >0.5, otherwise O₂ evolution occurs. This limits the charge capacity of this material to ~140 mA h g⁻¹.^[75]

Several other first-row (3d) transition metals also form layered LiMO₂ salts. Some can be directly prepared from lithium oxide and M₂O₃ (e.g. for M=Ti, V, Cr, Co, Ni), while others (M= Mn or Fe) can be prepared by ion exchange from NaMO₂. LiVO₂, LiMnO₂ and LiFeO₂ suffer from structural instabilities (including mixing between M and Li sites) due to a low energy difference between octahedral and tetrahedral environments for the metal ion M. For this reason, they are not used in lithium-ion batteries.^[75] However, Na⁺ and Fe³⁺ have sufficiently different sizes that NaFeO₂ can be used in sodium-ion batteries.^[76]

Similarly, LiCrO₂ shows reversible lithium (de)intercalation around 3.2 V with 170–270 mAh/g.^[77] However, its cycle life is short, because of disproportionation of Cr⁴⁺ followed by translocation of Cr⁶⁺ into tetrahedral sites.^[78] On the other hand, NaCrO₂ shows a much better cycling stability.^[79] LiTiO₂ shows Li+ (de)intercalation at a voltage of ~1.5 V, which is too low for a cathode material.

These problems leave LiCoO
₂ and LiNiO
₂ as the only practical layered oxide materials for lithium-ion battery cathodes. The cobalt-based cathodes show high theoretical specific (per-mass) charge capacity, high volumetric capacity, low self-discharge, high discharge voltage, and good cycling performance. Unfortunately, they suffer from a high cost of the material.^[80] For this reason, the current trend among lithium-ion battery manufacturers is to switch to cathodes with higher Ni content and lower Co content.^[81]

In addition to a lower (than cobalt) cost, nickel-oxide based materials benefit from the two-electron redox chemistry of Ni: in layered oxides comprising nickel (such as nickel-cobalt-manganese NCM and nickel-cobalt-aluminium oxides NCA), Ni cycles between the oxidation states +2 and +4 (in one step between +3.5 and +4.3 V),^[82][75] cobalt- between +2 and +3, while Mn (usually >20%) and Al (typically, only 5% is needed)^[83] remain in +4 and 3+, respectively. Thus increasing the Ni content increases the cyclable charge. For example, NCM111 shows 160 mAh/g, while LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) and LiNi_0.8Co_0.15Al_0.05O₂ (NCA) deliver a higher capacity of ~200 mAh/g.^[84]

It is worth mentioning so-called "lithium-rich" cathodes, that can be produced from traditional NCM (LiMO₂, where M=Ni, Co, Mn) layered cathode materials upon cycling them to voltages/charges corresponding to Li:M<0.5. Under such conditions a new semi-reversible redox transition at a higher voltage with ca. 0.4-0.8 electrons/metal site charge appears. This transition involves non-binding electron orbitals centered mostly on O atoms. Despite significant initial interest, this phenomenon did not result in marketable products because of the fast structural degradation (O2 evolution and lattice rearrangements) of such "lithium-rich" phases.^[85]

LiMn₂O₄ adopts a cubic lattice, which allows for three-dimensional lithium-ion diffusion.^[86] Manganese cathodes are attractive because manganese is less expensive than cobalt or nickel. The operating voltage of Li-LiMn₂O₄ battery is 4 V, and ca. one lithium per two Mn ions can be reversibly extracted from the tetrahedral sites, resulting in a practical capacity of <130 mA h g–1. However, Mn³⁺ is not a stable oxidation state, as it tends to disporportionate into insoluble Mn⁴⁺ and soluble Mn²⁺.^[80][87] LiMn₂O₄ can also intercalate more than 0.5 Li per Mn at a lower voltage around +3.0 V. However, this results in an irreversible phase transition due to Jahn-Teller distortion in Mn3+:t2g3eg1, as well as disproportionation and dissolution of Mn³⁺.

An important improvement of Mn spinel are related cubic structures of the LiMn_1.5Ni_0.5O₄ type, where Mn exists as Mn4+ and Ni cycles reversibly between the oxidation states +2 and +4.^[75] This materials show a reversible Li-ion capacity of ca. 135 mAh/g around 4.7 V. Although such high voltage is beneficial for increasing the specific energy of batteries, the adoption of such materials is currently hindered by the lack of suitable high-voltage electrolytes.^[88] In general, materials with a high nickel content are favored in 2023, because of the possibility of a 2-electron cycling of Ni between the oxidation states +2 and +4.

LiV₂O₄ (lithium vanadium oxide) operates as a lower (ca. +3.0 V) voltage than LiMn₂O₄, suffers from similar durability issues, is more expensive, and thus is not considered of practical interest.^[89]

Around 1980 Manthiram discovered that oxoanions (molybdates and tungstates in that particular case) cause a substantial positive shift in the redox potential of the metal-ion compared to oxides.^[90] In addition, these oxoanionic cathode materials offer better stability/safety than the corresponding oxides. However, they also suffer from poor electronic conductivity due to the long distance between redox-active metal centers, which slows down the electron transport. This necessitates the use of small (less than 200 nm) cathode particles and coating each particle with a layer of electronically-conducting carbon.^[91] This reduces the packing density of these materials.

Although numerous combinations of oxoanions (sulfate, phosphate, silicate) with various metals (mostly Mn, Fe, Co, Ni) have been studied, LiFePO4 is the only one that has been commercialized. Although it was originally used primarily for stationary energy storage due to its lower energy density compared to layered oxides,^[92] it has begun to be widely used in electric vehicles since the 2020s.^[93]

Negative electrode materials are traditionally constructed from graphite and other carbon materials, although newer silicon-based materials are being increasingly used (see Nanowire battery). In 2016, 89% of lithium-ion batteries contained graphite (43% artificial and 46% natural), 7% contained amorphous carbon (either soft carbon or hard carbon), 2% contained lithium titanate (LTO) and 2% contained silicon or tin-based materials.^[98]

These materials are used because they are abundant, electrically conducting and can intercalate lithium ions to store electrical charge with modest volume expansion (~10%).^[99] Graphite is the dominant material because of its low intercalation voltage and excellent performance. Various alternative materials with higher capacities have been proposed, but they usually have higher voltages, which reduces energy density.^[100] Low voltage is the key requirement for anodes; otherwise, the excess capacity is useless in terms of energy density.

As graphite is limited to a maximum capacity of 372 mAh/g^[47] much research has been dedicated to the development of materials that exhibit higher theoretical capacities and overcoming the technical challenges that presently encumber their implementation. The extensive 2007 Review Article by Kasavajjula et al.^[108] summarizes early research on silicon-based anodes for lithium-ion secondary cells. In particular, Hong Li et al.^[109] showed in 2000 that the electrochemical insertion of lithium ions in silicon nanoparticles and silicon nanowires leads to the formation of an amorphous Li-Si alloy. The same year, Bo Gao and his doctoral advisor, Professor Otto Zhou described the cycling of electrochemical cells with anodes comprising silicon nanowires, with a reversible capacity ranging from at least approximately 900 to 1500 mAh/g.^[110]

Diamond-like carbon coatings can increase retention capacity by 40% and cycle life by 400% for lithium based batteries.^[111]

To improve the stability of the lithium anode, several approaches to installing a protective layer have been suggested.^[112] Silicon is beginning to be looked at as an anode material because it can accommodate significantly more lithium ions, storing up to 10 times the electric charge, however this alloying between lithium and silicon results in significant volume expansion (ca. 400%),^[99] which causes catastrophic failure for the cell.^[113] Silicon has been used as an anode material but the insertion and extraction of ${\displaystyle {\ce {\scriptstyle Li+}}}$  can create cracks in the material. These cracks expose the Si surface to an electrolyte, causing decomposition and the formation of a solid electrolyte interphase (SEI) on the new Si surface (crumpled graphene encapsulated Si nanoparticles). This SEI will continue to grow thicker, deplete the available ${\displaystyle {\ce {\scriptstyle Li+}}}$ , and degrade the capacity and cycling stability of the anode.

In addition to carbon- and silicon- based anode materials for lithium-ion batteries, high-entropy metal oxide materials are being developed. These conversion (rather than intercalation) materials comprise an alloy (or subnanometer mixed phases) of several metal oxides performing different functions. For example, Zn and Co can act as electroactive charge-storing species, Cu can provide an electronically conducting support phase and MgO can prevent pulverization.^[114]

Liquid electrolytes in lithium-ion batteries consist of lithium salts, such as LiPF
₆, LiBF
₄ or LiClO
₄ in an organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate.^[115] A liquid electrolyte acts as a conductive pathway for the movement of cations passing from the negative to the positive electrodes during discharge. Typical conductivities of liquid electrolyte at room temperature (20 °C (68 °F)) are in the range of 10 mS/cm, increasing by approximately 30–40% at 40 °C (104 °F) and decreasing slightly at 0 °C (32 °F).^[116] The combination of linear and cyclic carbonates (e.g., ethylene carbonate (EC) and dimethyl carbonate (DMC)) offers high conductivity and solid electrolyte interphase (SEI)-forming ability. Organic solvents easily decompose on the negative electrodes during charge. When appropriate organic solvents are used as the electrolyte, the solvent decomposes on initial charging and forms a solid layer called the solid electrolyte interphase,^[117] which is electrically insulating, yet provides significant ionic conductivity. The interphase prevents further decomposition of the electrolyte after the second charge. For example, ethylene carbonate is decomposed at a relatively high voltage, 0.7 V vs. lithium, and forms a dense and stable interface.^[118] Composite electrolytes based on POE (poly(oxyethylene)) provide a relatively stable interface.^[119][120] It can be either solid (high molecular weight) and be applied in dry Li-polymer cells, or liquid (low molecular weight) and be applied in regular Li-ion cells. Room-temperature ionic liquids (RTILs) are another approach to limiting the flammability and volatility of organic electrolytes.^[121]

Recent advances in battery technology involve using a solid as the electrolyte material. The most promising of these are ceramics.^[122] Solid ceramic electrolytes are mostly lithium metal oxides, which allow lithium-ion transport through the solid more readily due to the intrinsic lithium. The main benefit of solid electrolytes is that there is no risk of leaks, which is a serious safety issue for batteries with liquid electrolytes.^[123] Solid ceramic electrolytes can be further broken down into two main categories: ceramic and glassy. Ceramic solid electrolytes are highly ordered compounds with crystal structures that usually have ion transport channels.^[124] Common ceramic electrolytes are lithium super ion conductors (LISICON) and perovskites. Glassy solid electrolytes are amorphous atomic structures made up of similar elements to ceramic solid electrolytes but have higher conductivities overall due to higher conductivity at grain boundaries.^[125] Both glassy and ceramic electrolytes can be made more ionically conductive by substituting sulfur for oxygen. The larger radius of sulfur and its higher ability to be polarized allow higher conductivity of lithium. This contributes to conductivities of solid electrolytes are nearing parity with their liquid counterparts, with most on the order of 0.1 mS/cm and the best at 10 mS/cm.^[126] An efficient and economic way to tune targeted electrolytes properties is by adding a third component in small concentrations, known as an additive.^[127] By adding the additive in small amounts, the bulk properties of the electrolyte system will not be affected whilst the targeted property can be significantly improved. The numerous additives that have been tested can be divided into the following three distinct categories: (1) those used for SEI chemistry modifications; (2) those used for enhancing the ion conduction properties; (3) those used for improving the safety of the cell (e.g. prevent overcharging).^{[citation needed]}

Electrolyte alternatives have also played a significant role, for example the lithium polymer battery. Polymer electrolytes are promising for minimizing the dendrite formation of lithium. Polymers are supposed to prevent short circuits and maintain conductivity.^[112]

The ions in the electrolyte diffuse because there are small changes in the electrolyte concentration. Linear diffusion is only considered here. The change in concentration c, as a function of time t and distance x, is

${\displaystyle {\frac {\partial c}{\partial t}}={\frac {D}{\varepsilon }}{\frac {\partial ^{2}c}{\partial x^{2}}}.}$ 

In this equation, D is the diffusion coefficient for the lithium ion. It has a value of 7.5×10⁻¹⁰ m²/s in the LiPF
₆ electrolyte. The value for ε, the porosity of the electrolyte, is 0.724.^[128]

Lithium-ion batteries may have multiple levels of structure. Small batteries consist of a single battery cell. Larger batteries connect cells in parallel into a module and connect modules in series and parallel into a pack. Multiple packs may be connected in series to increase the voltage.^[129]

Li-ion cells are available in various form factors, which can generally be divided into four types:^[130]

• Small cylindrical (solid body without terminals, such as those used in most e-bikes and most electric vehicle battery and older laptop batteries); they typically come in standard sizes.
• Large cylindrical (solid body with large threaded terminals)
• Flat or pouch (soft, flat body, such as those used in cell phones and newer laptops; these are lithium-ion polymer batteries.^[131]
• Rigid plastic case with large threaded terminals (such as electric vehicle traction packs)

Cells with a cylindrical shape are made in a characteristic "swiss roll" manner (known as a "jelly roll" in the US), which means it is a single long "sandwich" of the positive electrode, separator, negative electrode, and separator rolled into a single spool. The result is encased in a container. One advantage of cylindrical cells is faster production speed. One disadvantage can be a large radial temperature gradient at high discharge rates.

The absence of a case gives pouch cells the highest gravimetric energy density; however, many applications require containment to prevent expansion when their state of charge (SOC) level is high,^[132] and for general structural stability. Both rigid plastic and pouch-style cells are sometimes referred to as prismatic cells due to their rectangular shapes.^[133] Three basic battery types are used in 2020s-era electric vehicles: cylindrical cells (e.g., Tesla), prismatic pouch (e.g., from LG), and prismatic can cells (e.g., from LG, Samsung, Panasonic, and others).^[13]

Lithium-ion flow batteries have been demonstrated that suspend the cathode or anode material in an aqueous or organic solution.^[134][135]

As of 2014, the smallest Li-ion cell was pin-shaped with a diameter of 3.5 mm and a weight of 0.6 g, made by Panasonic.^[136] A coin cell form factor is available for LiCoO₂ cells, usually designated with a "LiR" prefix.^[137][138]

Batteries may be equipped with temperature sensors, heating/cooling systems, voltage regulator circuits, voltage taps, and charge-state monitors. These components address safety risks like overheating and short circuiting.^[139]

Lithium ion batteries are used in a multitude of applications from consumer electronics, toys, power tools and electric vehicles.^[140]

More niche uses include backup power in telecommunications applications. Lithium-ion batteries are also frequently discussed as a potential option for grid energy storage,^[141] although as of 2020, they were not yet cost-competitive at scale.^[142]

Because lithium-ion batteries can have a variety of positive and negative electrode materials, the energy density and voltage vary accordingly.

The open-circuit voltage is higher than in aqueous batteries (such as lead–acid, nickel–metal hydride and nickel–cadmium).^{[146][failed verification]} Internal resistance increases with both cycling and age,^[147] although this depends strongly on the voltage and temperature the batteries are stored at.^[148] Rising internal resistance causes the voltage at the terminals to drop under load, which reduces the maximum current draw. Eventually, increasing resistance will leave the battery in a state such that it can no longer support the normal discharge currents requested of it without unacceptable voltage drop or overheating.

Batteries with a lithium iron phosphate positive and graphite negative electrodes have a nominal open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese cobalt (NMC) oxide positives with graphite negatives have a 3.7 V nominal voltage with a 4.2 V maximum while charging. The charging procedure is performed at constant voltage with current-limiting circuitry (i.e., charging with constant current until a voltage of 4.2 V is reached in the cell and continuing with a constant voltage applied until the current drops close to zero). Typically, the charge is terminated at 3% of the initial charge current. In the past, lithium-ion batteries could not be fast-charged and needed at least two hours to fully charge. Current-generation cells can be fully charged in 45 minutes or less. In 2015 researchers demonstrated a small 600 mAh capacity battery charged to 68 percent capacity in two minutes and a 3,000 mAh battery charged to 48 percent capacity in five minutes. The latter battery has an energy density of 620 W·h/L. The device employed heteroatoms bonded to graphite molecules in the anode.^[149]

Performance of manufactured batteries has improved over time. For example, from 1991 to 2005 the energy capacity per price of lithium-ion batteries improved more than ten-fold, from 0.3 W·h per dollar to over 3 W·h per dollar.^[150] In the period from 2011 to 2017, progress has averaged 7.5% annually.^[151] Overall, between 1991 and 2018, prices for all types of lithium-ion cells (in dollars per kWh) fell approximately 97%.^[152] Over the same time period, energy density more than tripled.^[152] Efforts to increase energy density contributed significantly to cost reduction.^[153] Energy density can also be increased by improvements in the chemistry if the cell, for instance, by full or partial replacement of graphite with silicon. Silicon anodes enhanced with graphene nanotubes to eliminate the premature degradation of silicon open the door to reaching record-breaking battery energy density of up to 350 Wh/kg and lowering EV prices to be competitive with ICEs.^[154]

Differently sized cells with similar chemistry can also have different energy densities. The 21700 cell has 50% more energy than the 18650 cell, and the bigger size reduces heat transfer to its surroundings.^[145]

The table below shows the result of an experimental evaluation of a "high-energy" type 3.0 Ah 18650 NMC cell in 2021, round-trip efficiency which compared the energy going into the cell and energy extracted from the cell from 100% (4.2v) SoC to 0% SoC (cut off 2.0v). A roundtrip efficiency is the percent of energy that can be used relative to the energy that went into charging the battery.^[155]

Characterization of a cell in a different experiment in 2017 reported round-trip efficiency of 85.5% at 2C and 97.6% at 0.1C^[156]

The lifespan of a lithium-ion battery is typically defined as the number of full charge-discharge cycles to reach a failure threshold in terms of capacity loss or impedance rise. Manufacturers' datasheet typically uses the word "cycle life" to specify lifespan in terms of the number of cycles to reach 80% of the rated battery capacity.^[157] Simply storing lithium-ion batteries in the charged state also reduces their capacity (the amount of cyclable Li⁺) and increases the cell resistance (primarily due to the continuous growth of the solid electrolyte interface on the anode). Calendar life is used to represent the whole life cycle of battery involving both the cycle and inactive storage operations. Battery cycle life is affected by many different stress factors including temperature, discharge current, charge current, and state of charge ranges (depth of discharge).^[158][159] Batteries are not fully charged and discharged in real applications such as smartphones, laptops and electric cars and hence defining battery life via full discharge cycles can be misleading. To avoid this confusion, researchers sometimes use cumulative discharge^[158] defined as the total amount of charge (Ah) delivered by the battery during its entire life or equivalent full cycles,^[159] which represents the summation of the partial cycles as fractions of a full charge-discharge cycle. Battery degradation during storage is affected by temperature and battery state of charge (SOC) and a combination of full charge (100% SOC) and high temperature (usually > 50 °C) can result in a sharp capacity drop and gas generation.^[160] Multiplying the battery cumulative discharge by the rated nominal voltage gives the total energy delivered over the life of the battery. From this one can calculate the cost per kWh of the energy (including the cost of charging).

Over their lifespan batteries degrade gradually leading to reduced cyclable charge (a.k.a. Ah capacity) and increased resistance (the latter translates into a lower operating cell voltage).^[161]

Several degradation processes occur in lithium-ion batteries, some during cycling, some during storage, and some all the time:^{[162][163][161]} Degradation is strongly temperature-dependent: degradation at room temperature is minimal but increases for batteries stored or used in high temperature (usually > 35 °C) or low temperature (usually < 5 °C) environments.^[164] High charge levels also hasten capacity loss.^[165] Frequent over-charging (> 90%) and over-discharging (< 10%) may also hasten capacity loss.

In a study, scientists provided 3D imaging and model analysis to reveal main causes, mechanics, and potential mitigations of the problematic degradation of the batteries over charge cycles. They found "[p]article cracking increases and contact loss between particles and carbon-binder domain are observed to correlate with the cell degradation" and indicates that "the reaction heterogeneity within the thick cathode caused by the unbalanced electron conduction is the main cause of the battery degradation over cycling".^{[166][167][additional citation(s) needed]}

The most common degradation mechanisms in lithium-ion batteries include:^[168]

1. Reduction of the organic carbonate electrolyte at the anode, which results in the growth of Solid Electrolyte Interface (SEI), where Li⁺ ions get irreversibly trapped, i.e. loss of lithium inventory. This shows as increased ohmic impedance and reduced Ah charge. At constant temperature, the SEI film thickness (and therefore, the SEI resistance and the loss in cyclable Li⁺) increases as a square root of the time spent in the charged state. The number of cycles is not a useful metric in characterizing this degradation pathway. Under high temperatures or in the presence of a mechanical damage the electrolyte reduction can proceed explosively.
2. Lithium metal plating also results in the loss of lithium inventory (cyclable Ah charge), as well as internal short-circuiting and ignition of a battery. Once Li plating commences during cycling, it results in larger slopes of capacity loss per cycle and resistance increase per cycle. This degradation mechanism become more prominent during fast charging and low temperatures.
3. Loss of the (negative or positive) electroactive materials due to dissolution (e.g. of Mn³⁺ species), cracking, exfoliation, detachment or even simple regular volume change during cycling. It shows up as both charge and power fade (increased resistance). Both positive and negative electrode materials are subject to fracturing due to the volumetric strain of repeated (de)lithiation cycles.
4. Structural degradation of cathode materials, such as Li⁺/Ni²⁺ cation mixing in nickel-rich materials. This manifests as “electrode saturation", loss of cyclable Ah charge and as a "voltage fade".
5. Other material degradations. Negative copper current collector is particularly prone to corrosion/dissolution at low cell voltages. PVDF binder also degrades, causing the detachment of the electroactive materials, and the loss of cyclable Ah charge.

These are shown in the figure on the right. A change from one main degradation mechanism to another appears as a knee (slope change) in the capacity vs. cycle number plot.^[168]

Most studies of lithium-ion battery aging have been done at elevated (50–60 °C) temperatures in order to complete the experiments sooner. Under these storage conditions, fully charged nickel-cobalt-aluminum and lithium-iron phosphate cells lose ca. 20% of their cyclable charge in 1–2 years. It is believed that the aforementioned anode aging is the most important degradation pathway in these cases. On the other hand, manganese-based cathodes show a (ca. 20–50%) faster degradation under these conditions, probably due to the additional mechanism of Mn ion dissolution.^[163] At 25 °C the degradation of lithium-ion batteries seems to follow the same pathway(s) as the degradation at 50 °C, but with half the speed.^[163] In other words, based on the limited extrapolated experimental data, lithium-ion batteries are expected to lose irreversibly ca. 20% of their cyclable charge in 3–5 years or 1000–2000 cycles at 25 °C.^[168] Lithium-ion batteries with titanate anodes do not suffer from SEI growth, and last longer (>5000 cycles) than graphite anodes. However, in complete cells other degradation mechanisms (i.e. the dissolution of Mn³⁺ and the Ni²⁺/Li⁺ place exchange, decomposition of PVDF binder and particle detachment) show up after 1000–2000 days, and the use titanate anode does not improve full cell durability in practice.

A more detailed description of some of these mechanisms is provided below:

The IEEE standard 1188–1996 recommends replacing lithium-ion batteries in an electric vehicle, when their charge capacity drops to 80% of the nominal value.^[191] In what follows, we shall use the 20% capacity loss as a comparison point between different studies. We shall note, nevertheless, that the linear model of degradation (the constant % of charge loss per cycle or per calendar time) is not always applicable, and that a “knee point”, observed as a change of the slope, and related to the change of the main degradation mechanism, is often observed.^[192]

The problem of lithium-ion battery safety has been recognized even before these batteries were first commercially released in 1991. The two main reasons for lithium-ion battery fires and explosions are related to processes on the negative electrode (cathode). During a normal battery charge lithium ions intercalate into graphite. However, if the charge is forced to go too fast (or at a too low temperature) lithium metal starts plating on the anode, and the resulting dendrites can penetrate the battery separator, internally short-circuit the cell, resulting in high electric current, heating and ignition. In other mechanism, an explosive reaction between the charge anode material (LiC₆) and the solvent (liquid organic carbonate) occurs even at open circuit, provided that the anode temperature exceeds a certain threshold above 70 °C.^[193]

Nowadays, all reputable manufacturers employ at least two safety devices in all their lithium-ion batteries of an 18650 format or larger: a current interrupt (CID) device and a positive temperature coefficient (PTC) device. The CID comprises two metal disks, that make an electric contact with each other. When pressure inside the cell increases, the distance between the two disks increases too and they lose the electric contact with each other, thus terminating the flow of electric current through the battery. The PTC device is made of an electrically conducting polymer. When the current going through the PTC device increases, the polymer gets hot, and its electric resistance rises sharply, thus reducing the current through the battery.^[194]

Lithium-ion batteries can be a safety hazard since they contain a flammable electrolyte and may become pressurized if they become damaged. A battery cell charged too quickly could cause a short circuit, leading to overheating, explosions, and fires.^[195] A Li-ion battery fire can be started due to (1) thermal abuse, e.g. poor cooling or external fire, (2) electrical abuse, e.g. overcharge or external short circuit, (3) mechanical abuse, e.g. penetration or crash, or (4) internal short circuit, e.g. due to manufacturing flaws or aging.^[196][197] Because of these risks, testing standards are more stringent than those for acid-electrolyte batteries, requiring both a broader range of test conditions and additional battery-specific tests, and there are shipping limitations imposed by safety regulators.^{[65][198][199]} There have been battery-related recalls by some companies, including the 2016 Samsung Galaxy Note 7 recall for battery fires.^[200][201]

Lithium-ion batteries have a flammable liquid electrolyte.^[202] A faulty battery can cause a serious fire.^[195] Faulty chargers can affect the safety of the battery because they can destroy the battery's protection circuit. While charging at temperatures below 0 °C, the negative electrode of the cells gets plated with pure lithium, which can compromise the safety of the whole pack.

Short-circuiting a battery will cause the cell to overheat and possibly to catch fire.^[203] Smoke from thermal runaway in a Li-ion battery is both flammable and toxic.^[204] The fire energy content (electrical + chemical) of cobalt-oxide cells is about 100 to 150 kJ/(A·h), most of it chemical.^{[unreliable source?][205]}

Around 2010, large lithium-ion batteries were introduced in place of other chemistries to power systems on some aircraft; as of January 2014^[update], there had been at least four serious lithium-ion battery fires, or smoke, on the Boeing 787 passenger aircraft, introduced in 2011, which did not cause crashes but had the potential to do so.^[206][207] UPS Airlines Flight 6 crashed in Dubai after its payload of batteries spontaneously ignited.

To reduce fire hazards, research projects are intended to develop non-flammable electrolytes.^{[citation needed]}

If a lithium-ion battery is damaged, crushed, or is subjected to a higher electrical load without having overcharge protection, then problems may arise. External short circuit can trigger a battery explosion.^[208] Such incidents can occur when lithium-ion batteries are not disposed of through the appropriate channels, but are thrown away with other waste. The way they are treated by recycling companies can damage them and cause fires, which in turn can lead to large-scale conflagrations. Twelve such fires were recorded in Swiss recycling facilities in 2023.^[209]

If overheated or overcharged, Li-ion batteries may suffer thermal runaway and cell rupture.^[210][211] During thermal runaway, internal degradation and oxidization processes can keep cell temperatures above 500 °C, with the possibility of igniting secondary combustibles, as well as leading to leakage, explosion or fire in extreme cases.^[212] To reduce these risks, many lithium-ion cells (and battery packs) contain fail-safe circuitry that disconnects the battery when its voltage is outside the safe range of 3–4.2 V per cell,^[213][74] or when overcharged or discharged. Lithium battery packs, whether constructed by a vendor or the end-user, without effective battery management circuits are susceptible to these issues. Poorly designed or implemented battery management circuits also may cause problems; it is difficult to be certain that any particular battery management circuitry is properly implemented.

Lithium-ion cells are susceptible to stress by voltage ranges outside of safe ones between 2.5 and 3.65/4.1/4.2 or 4.35 V (depending on the components of the cell). Exceeding this voltage range results in premature aging and in safety risks due to the reactive components in the cells.^[214] When stored for long periods the small current draw of the protection circuitry may drain the battery below its shutoff voltage; normal chargers may then be useless since the battery management system (BMS) may retain a record of this battery (or charger) "failure". Many types of lithium-ion cells cannot be charged safely below 0 °C,^[215] as this can result in plating of lithium on the anode of the cell, which may cause complications such as internal short-circuit paths.^{[citation needed]}

Other safety features are required^{[by whom?]} in each cell:^[213]

• Shut-down separator (for overheating)
• Tear-away tab (for internal pressure relief)
• Vent (pressure relief in case of severe outgassing)
• Thermal interrupt (overcurrent/overcharging/environmental exposure)

These features are required because the negative electrode produces heat during use, while the positive electrode may produce oxygen. However, these additional devices occupy space inside the cells, add points of failure, and may irreversibly disable the cell when activated. Further, these features increase costs compared to nickel metal hydride batteries, which require only a hydrogen/oxygen recombination device and a back-up pressure valve.^[74] Contaminants inside the cells can defeat these safety devices. Also, these features can not be applied to all kinds of cells, e.g., prismatic high current cells cannot be equipped with a vent or thermal interrupt. High current cells must not produce excessive heat or oxygen, lest there be a failure, possibly violent. Instead, they must be equipped with internal thermal fuses which act before the anode and cathode reach their thermal limits.^[216]

Replacing the lithium cobalt oxide positive electrode material in lithium-ion batteries with a lithium metal phosphate such as lithium iron phosphate (LFP) improves cycle counts, shelf life and safety, but lowers capacity. As of 2006, these safer lithium-ion batteries were mainly used in electric cars and other large-capacity battery applications, where safety is critical.^[217] In 2016, an LFP-based energy storage system was chosen to be installed in Paiyun Lodge on Mt.Jade (Yushan) (the highest lodge in Taiwan). As of June 2024, the system was still operating safely.^[218]

In 2006, approximately 10 million Sony batteries used in Dell, Sony, Apple, Lenovo, Panasonic, Toshiba, Hitachi, Fujitsu and Sharp laptops were recalled. The batteries were found to be susceptible to internal contamination by metal particles during manufacture. Under some circumstances, these particles could pierce the separator, causing a dangerous short circuit.^[219]

IATA estimates that over a billion lithium metal and lithium-ion cells are flown each year.^[205] Some kinds of lithium batteries may be prohibited aboard aircraft because of the fire hazard.^[220][221] Some postal administrations restrict air shipping (including EMS) of lithium and lithium-ion batteries, either separately or installed in equipment.

In 2023, most commercial Li-ion batteries employed alkylcarbonate solvent(s) to assure the formation solid electrolyte interface on the negative electrode. Since such solvents are readily flammable, there has been active research to replace them with non-flammable solvents or to add fire suppressants. Another source of hazard is hexafluorophosphate anion, which is needed to passitivate the negative current collector made of aluminium. Hexafluorophosphate reacts with water and releases volatile and toxic hydrogen fluoride. Efforts to replace hexafluorophosphate have been less successful.

In the 1990s, the United States was the World’s largest miner of lithium minerals, contributing to 1/3 of the total production. By 2010 Chile replaced the USA the leading miner, thanks to the development of lithium brines in Salar de Atacama. By 2024, Australia and China joined Chile as the top 3 miners. Li-ion battery production is also heavily concentrated, with 60% coming from China in 2024.^[222]

Extraction of lithium, nickel, and cobalt, manufacture of solvents, and mining byproducts present significant environmental and health hazards.^{[224][225][226]} Lithium extraction can be fatal to aquatic life due to water pollution.^[227] It is known to cause surface water contamination, drinking water contamination, respiratory problems, ecosystem degradation and landscape damage.^[224] It also leads to unsustainable water consumption in arid regions (1.9 million liters per ton of lithium).^[224] Massive byproduct generation of lithium extraction also presents unsolved problems, such as large amounts of magnesium and lime waste.^[228]

Lithium mining takes place in North and South America, Asia, South Africa, Australia, and China.^[229]

Cobalt for Li-ion batteries is largely mined in the Congo (see also Mining industry of the Democratic Republic of the Congo). Open-pit cobalt mining has led to deforestation and habitat destruction in the Democratic Republic of Congo.^[230]

Open-pit nickel mining has led to environmental degradation and pollution in developing countries such as the Philippines and Indonesia.^[231][232] In 2024, nickel mining and processing was one of the main causes of deforestation in Indonesia.^[233][234]

Manufacturing a kg of Li-ion battery takes about 67 megajoule (MJ) of energy.^[235][236] The global warming potential of lithium-ion batteries manufacturing strongly depends on the energy source used in mining and manufacturing operations, and is difficult to estimate, but one 2019 study estimated 73 kg CO2e/kWh.^[237] Effective recycling can reduce the carbon footprint of the production significantly.^[238]

Li-ion battery elements including iron, copper, nickel and cobalt are considered safe for incinerators and landfills.^{[239][citation needed]} These metals can be recycled,^[240][241] usually by burning away the other materials,^[242] but mining generally remains cheaper than recycling;^[243] recycling may cost $3/kg,^[244] and in 2019 less than 5% of lithium-ion batteries were being recycled.^[245] Since 2018, the recycling yield was increased significantly, and recovering lithium, manganese, aluminum, the organic solvents of the electrolyte, and graphite is possible at industrial scales.^[246] The most expensive metal involved in the construction of the cell is cobalt. Lithium is less expensive than other metals used and is rarely recycled,^[242] but recycling could prevent a future shortage.^[240]

Accumulation of battery waste presents technical challenges and health hazards.^[247] Since the environmental impact of electric cars is heavily affected by the production of lithium-ion batteries, the development of efficient ways to repurpose waste is crucial.^[245] Recycling is a multi-step process, starting with the storage of batteries before disposal, followed by manual testing, disassembling, and finally the chemical separation of battery components. Re-use of the battery is preferred over complete recycling as there is less embodied energy in the process. As these batteries are a lot more reactive than classical vehicle waste like tire rubber, there are significant risks to stockpiling used batteries.^[248]

The pyrometallurgical method uses a high-temperature furnace to reduce the components of the metal oxides in the battery to an alloy of Co, Cu, Fe, and Ni. This is the most common and commercially established method of recycling and can be combined with other similar batteries to increase smelting efficiency and improve thermodynamics. The metal current collectors aid the smelting process, allowing whole cells or modules to be melted at once.^[249] The product of this method is a collection of metallic alloy, slag, and gas. At high temperatures, the polymers used to hold the battery cells together burn off and the metal alloy can be separated through a hydrometallurgical process into its separate components. The slag can be further refined or used in the cement industry. The process is relatively risk-free and the exothermic reaction from polymer combustion reduces the required input energy. However, in the process, the plastics, electrolytes, and lithium salts will be lost.^[250]

This method involves the use of aqueous solutions to remove the desired metals from the cathode. The most common reagent is sulfuric acid.^[251] Factors that affect the leaching rate include the concentration of the acid, time, temperature, solid-to-liquid-ratio, and reducing agent.^[252] It is experimentally proven that H₂O₂ acts as a reducing agent to speed up the rate of leaching through the reaction:^{[citation needed]}

2 LiCoO_{2 (s)} + 3 H₂SO₄ + H₂O₂ → 2 CoSO_{4 (aq)} + Li₂SO₄ + 4 H₂O + O₂

Once leached, the metals can be extracted through precipitation reactions controlled by changing the pH level of the solution. Cobalt, the most expensive metal, can then be recovered in the form of sulfate, oxalate, hydroxide, or carbonate. [75] More recently recycling methods experiment with the direct reproduction of the cathode from the leached metals. In these procedures, concentrations of the various leached metals are premeasured to match the target cathode and then the cathodes are directly synthesized.^[253]

The main issues with this method, however, is that a large volume of solvent is required and the high cost of neutralization. Although it's easy to shred up the battery, mixing the cathode and anode at the beginning complicates the process, so they will also need to be separated. Unfortunately, the current design of batteries makes the process extremely complex and it is difficult to separate the metals in a closed-loop battery system. Shredding and dissolving may occur at different locations.^[254]

Direct recycling is the removal of the cathode or anode from the electrode, reconditioned, and then reused in a new battery. Mixed metal-oxides can be added to the new electrode with very little change to the crystal morphology. The process generally involves the addition of new lithium to replenish the loss of lithium in the cathode due to degradation from cycling. Cathode strips are obtained from the dismantled batteries, then soaked in NMP, and undergo sonication to remove excess deposits. It is treated hydrothermally with a solution containing LiOH/Li₂SO₄ before annealing.^[255]

This method is extremely cost-effective for noncobalt-based batteries as the raw materials do not make up the bulk of the cost. Direct recycling avoids the time-consuming and expensive purification steps, which is great for low-cost cathodes such as LiMn₂O₄ and LiFePO₄. For these cheaper cathodes, most of the cost, embedded energy, and carbon footprint is associated with the manufacturing rather than the raw material.^[256] It is experimentally shown that direct recycling can reproduce similar properties to pristine graphite.

The drawback of the method lies in the condition of the retired battery. In the case where the battery is relatively healthy, direct recycling can cheaply restore its properties. However, for batteries where the state of charge is low, direct recycling may not be worth the investment. The process must also be tailored to the specific cathode composition, and therefore the process must be configured to one type of battery at a time.^[257] Lastly, in a time with rapidly developing battery technology, the design of a battery today may no longer be desirable a decade from now, rendering direct recycling ineffective.

Physical materials separation recovered materials by mechanical crushing and exploiting physical properties of different components such as particle size, density, ferromagnetism and hydrophobicity. Copper, aluminum and steel casing can be recovered by sorting. The remaining materials, called "black mass", which is composed of nickel, cobalt, lithium and manganese, need a secondary treatment to recover.^[258]

For the biological metals reclamation or bio-leaching, the process uses microorganisms to digest metal oxides selectively. Then, recyclers can reduce these oxides to produce metal nanoparticles. Although bio-leaching has been used successfully in the mining industry, this process is still nascent to the recycling industry and plenty of opportunities exists for further investigation.^[258]

Electrolyte recycling consists of two phases. The collection phase extracts the electrolyte from the spent Li-ion battery. This can be achieved through mechanical processes, distillation, freezing, solvent extraction, and supercritical fluid extraction. Due to the volatility, flammability, and sensitivity of the electrolyte, the collection process poses a greater difficulty than the collection process for other components of a Li-ion battery. The next phase consists of separation/electrolyte regeneration. Separation consists of isolating the individual components of the electrolyte. This approach is often used for the direct recovery of the Li salts from the organic solvents. In contrast, regeneration of the electrolyte aims to preserve the electrolyte composition by removing impurities which can be achieved through filtration methods.^[259][260]

The recycling of the electrolytes, which consists 10-15 wt.% of the Li-ion battery, provides both an economic and environmental benefits. These benefits include the recovery of the valuable Li-based salts and the prevention of hazardous compounds, such as volatile organic compounds (VOCs) and carcinogens, being released into the environment.

Compared to electrode recycling, less focus is placed on recycling the electrolyte of Li-ion batteries which can be attributed to lower economic benefits and greater process challenges. Such challenges can include the difficulty associated with recycling different electrolyte compositions,^[261] removing side products accumulated from electrolyte decomposition during its runtime,^[262] and removal of electrolyte adsorbed onto the electrodes.^[263] Due to these challenges, current pyrometallurgical methods of Li-ion battery recycling forgo electrolyte recovery, releasing hazardous gases upon heating. However, due to high energy consumption and environmental impact, future recycling methods are being directed away from this approach.^[264]

Extraction of raw materials for lithium-ion batteries may present dangers to local people, especially land-based indigenous populations.^[265]

Cobalt sourced from the Democratic Republic of the Congo is often mined by workers using hand tools with few safety precautions, resulting in frequent injuries and deaths.^[266] Pollution from these mines has exposed people to toxic chemicals that health officials believe to cause birth defects and breathing difficulties.^[267] Human rights activists have alleged, and investigative journalism reported confirmation,^[268][269] that child labor is used in these mines.^[270]

A study of relationships between lithium extraction companies and indigenous peoples in Argentina indicated that the state may not have protected indigenous peoples' right to free prior and informed consent, and that extraction companies generally controlled community access to information and set the terms for discussion of the projects and benefit sharing.^[271]

Development of the Thacker Pass lithium mine in Nevada, USA has met with protests and lawsuits from several indigenous tribes who have said they were not provided free prior and informed consent and that the project threatens cultural and sacred sites.^[272] Links between resource extraction and missing and murdered indigenous women have also prompted local communities to express concerns that the project will create risks to indigenous women.^[273] Protestors have been occupying the site of the proposed mine since January, 2021.^[274][275]

Researchers are actively working to improve the power density, safety, cycle durability (battery life), recharge time, cost, flexibility, and other characteristics, as well as research methods and uses, of these batteries. All-solid-state batteries are being researched as a breakthrough in technological barriers. Currently, all-solid-state batteries are expected to be the most promising next-generation battery, and various companies are working to popularize them.

• Anode-free battery
• Blade battery
• Borate oxalate
• Comparison of commercial battery types
• European Battery Alliance
• Flow battery
• Nanowire battery
• Sodium-ion battery
• Solid-state battery
• Thin-film lithium-ion battery
• VRLA battery
• Ultium

• Lithium-ion Battery at the Encyclopædia Britannica.
• List of World's Largest Lithium-ion Battery Factories (2020).
• Energy Storage Safety at National Renewable Energy Laboratory (NREL).
• New More Efficient Lithium-ion Batteries The New York Times. September 2021.
• NREL Innovation Improves Safety of Electric Vehicle Batteries, NREL, October 2015.
• Degradation Mechanisms and Lifetime Prediction for Lithium-Ion Batteries, NREL, July 2015.
• Impact of Temperature Extremes on Large Format Li-ion Batteries for Vehicle Applications, NREL, March 2013.