Electrochemistry

Lithium-Ion Battery

3.7 V cell with graphite anode + LiCoO2/NMC/LFP cathode — Whittingham, Goodenough, Yoshino 2019 Nobel

A lithium-ion cell is a rechargeable electrochemical device that shuttles Li+ ions between a graphite anode and a transition-metal oxide cathode (LiCoO2, NMC, NCA, or LFP) through a non-aqueous electrolyte. Nominal cell voltage is 3.7 V (peaking at 4.2 V on full charge), and modern cells reach gravimetric energy densities of 250-300 Wh/kg — five to six times lead-acid (~50 Wh/kg). M. Stanley Whittingham demonstrated reversible Li intercalation in TiS2 in 1976; John Goodenough discovered LiCoO2 as a 4 V cathode in 1980; Akira Yoshino paired LiCoO2 with petroleum-coke carbon in 1985 to make the first commercial-prototype Li-ion cell. The three shared the 2019 Nobel Prize in Chemistry.

  • Nominal voltage3.7 V
  • Charge cutoff4.2 V
  • Energy density250-300 Wh/kg
  • AnodeGraphite (LiC6)
  • CathodesLiCoO2, NMC, NCA, LFP
  • InventorsWhittingham, Goodenough, Yoshino — 2019 Nobel

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Why lithium-ion batteries matter

  • Five to six times more energy dense than lead-acid. Modern Li-ion packs deliver 250-300 Wh/kg; lead-acid (deep-cycle, AGM, gel) tops out near 50 Wh/kg. A Tesla Model Y 75 kWh pack at ~270 Wh/kg weighs 480 kg; the same energy in lead-acid would weigh ~1.5 tonnes — four times the curb weight contribution.
  • 3.7 V matches consumer electronics. A single Li-ion cell directly powers most CMOS and mobile-radio circuits without DC-DC step-up; alkaline (1.5 V) and NiMH (1.2 V) require boost conversion that costs 5-10% efficiency. This is why USB-PD, drones, laptops, and phones all use Li-ion natively.
  • Cycle life of 1000-3000+ cycles. A typical Tesla NMC pack delivers 1500 cycles to 80% capacity (about 300,000 km of driving). LFP cells (used in standard-range Teslas, BYD, CATL stationary storage) achieve 3000-7000 cycles, dominating utility-scale energy storage applications.
  • Self-discharge under 5% per month. Lead-acid loses 5-15% per month; NiCd 20%; Li-ion only 1-3%. A Li-ion device left unused for a year still has ~70-90% of its initial charge, making the chemistry uniquely suited to standby applications.
  • Coulombic efficiency >99.9% per cycle. The fraction of Li+ that returns to the cathode on discharge versus the amount sent during charge. High coulombic efficiency is what makes long cycle life possible — at 99.0% you'd lose ~63% capacity in 100 cycles; at 99.95% you lose only ~5%.
  • Cost dropped from $1200/kWh in 2010 to ~$110/kWh in 2024. A factor-of-11 decline in 14 years (BloombergNEF, "battery price survey 2024"). LFP cells now hit ~$80/kWh at the cell level. The roadmap to $50/kWh by 2030 — the price at which EVs reach unsubsidized parity with gasoline cars — depends on continued cathode and pack-integration improvements.
  • Enabled mass-market EVs and grid storage. Tesla shipped the Roadster in 2008, the Model S in 2012, and is now (2026) one of >50 manufacturers delivering production EVs. Grid-scale Li-ion storage (e.g., Tesla Megapack, BYD Cube) deployed >150 GWh globally in 2024 alone, providing frequency regulation, peak shaving, and renewable firming.

Common misconceptions

  • "Memory effect" applies to Li-ion. No — that's NiCd. Li-ion has effectively zero memory effect; partial charges and discharges do not reduce capacity. The lingering "always discharge fully before recharging" advice from the 1990s NiCd era is actively harmful for Li-ion (deep discharges accelerate aging).
  • Lithium metal is the anode. No, graphite is. Pure lithium metal anodes plate dendrites that short the cell — the failure mode that killed early-1980s rechargeable Li metal phone batteries (the "Moli Energy fires"). Li metal is the anode in primary (non-rechargeable) Li cells and in solid-state research, not in commercial Li-ion.
  • Higher voltage means more energy stored. Energy = voltage × charge capacity. NMC at 4.2 V max sounds higher than LFP at 3.65 V max, but volumetric Li capacity matters more. LFP's energy density is ~30% lower because of the lower voltage AND lower volumetric capacity, not voltage alone.
  • Charging in the cold is fine. Below 0°C, Li+ diffusion in graphite is slow enough that incoming Li metallizes on the anode surface instead of intercalating, forming dendrites. Most EV BMS units block charging below 0°C and use heaters; phones limit fast-charging current below ~5°C. Discharging cold is mostly OK, just lower capacity.
  • Storing fully charged extends life. Storage at 100% SOC and high temperature accelerates calendar aging. Optimal long-term storage is 40-60% SOC at 15-25°C; a year at 100%/40°C can lose 25%+ capacity, while 50%/15°C loses ~3%.
  • "Solid-state batteries are coming next year." The phrase has been used every year since ~2010. As of 2026 there are pilot lines and small-form prototype cells (e.g., Samsung SDI, Solid Power, QuantumScape, Toyota); EV-scale mass production is widely projected for 2027-2030 if dendrite suppression and manufacturing scale challenges are solved.

How a Li-ion cell works

A modern Li-ion cell is a sandwich. The negative electrode is graphite coated on a copper current collector; the positive electrode is a layered transition-metal oxide (most commonly LiNi0.8Mn0.1Co0.1O2, abbreviated NMC811, or LiFePO4, abbreviated LFP) coated on aluminum. Between them is a porous polymer separator (polyethylene/polypropylene composite, ~15-25 μm thick) soaked in electrolyte: 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), with additives like vinylene carbonate (VC) and fluoroethylene carbonate (FEC) to stabilize the SEI.

On charge, an external power source pumps electrons from cathode to anode and Li+ follows through the electrolyte. The cathode reaction LiCoO2 → Li(1-x)CoO2 + xLi+ + xe- deintercalates lithium from layered planes (Co3+ oxidizes to Co4+); the anode reaction xLi+ + xe- + 6C → LixC6 intercalates lithium between graphene layers (each Li donates an electron to the π system). At full charge x ≈ 1, giving stoichiometric LiC6 at ≈ 0.1 V vs Li/Li+; the cathode sits at ~4.2 V vs Li/Li+; cell voltage is the difference, ~4.1-4.2 V. On discharge, the reactions reverse and current flows through the load.

The SEI (Solid Electrolyte Interphase) is the unsung hero. On the first charge, electrolyte components reduce irreversibly at the graphite surface to form a thin (10-50 nm) layer of Li2CO3, lithium alkyl carbonates, LiF, and polymer fragments. The SEI is electronically insulating but Li+ conductive — it lets Li+ through while preventing further electrolyte breakdown. SEI formation consumes 5-15% of the cell's nominal capacity on first cycle (the "formation loss") and is the reason factories run slow first-charge cycles. SEI cracking and regrowth during cycling — driven by volume changes, fast charging, or Li plating at low temperature — is the dominant capacity-fade mechanism over the cell's life.

Comparison: rechargeable battery chemistries

ChemistryVoltageEnergy (Wh/kg)Cycle lifeNotes
Li-ion (NMC811/NCA)3.6-4.2 V250-300800-2000EVs, laptops; thermal runaway risk
Li-ion (LFP)3.0-3.65 V140-1803000-7000+Cobalt-free, very stable; grid storage
Solid-state Li-metal3.7-4.4 V~400 (proj.)500-1500 (lab)Pilot lines 2025-2026, EV ETA 2027-2030
Sodium-ion (Na-ion)3.0-3.5 V120-1602000-5000CATL 2023 production; cheap raw materials
Lead-acid2.0 V/cell30-50200-1000SLI starters, cheap stationary; 1859 invention
Ni-MH1.2 V60-110500-1000Hybrids (Prius), eneloop AAs
Ni-Cd1.2 V40-60500-1500Memory effect, toxic Cd; mostly retired
Vanadium redox flow1.4 V15-2510,000+Decoupled energy/power; long-duration grid

Applications and examples

  • Electric vehicles. A Tesla Model 3 Long Range pack contains ~4416 cylindrical 21700 cells (NMC or NCA) wired in 96 series × 46 parallel for 76 kWh at ~340 V nominal, weighing ~470 kg. The Tesla Megapack uses LFP prismatic cells for 3.9 MWh per unit at the utility scale.
  • Smartphones. A typical iPhone 15 Pro battery is a 3.27 Wh single-cell Li-polymer (LiCoO2 cathode, graphite-Si anode), ~205 g/Wh; ~10 hours of mixed use. Fast charging at 27 W now safely fills 0-50% in ~30 minutes thanks to ceramic-coated separators and bilayer SEI additives.
  • Grid frequency regulation. Hornsdale Power Reserve (Australia, Tesla 2017) was the first 100 MW Li-ion installation; it pays back its capital cost in ~3-5 years through frequency-control ancillary services. As of 2026 there are dozens of 100+ MW Li-ion sites globally.
  • Power tools and drones. 18650 NMC cells at 3.7 V × 3.5 Ah deliver short bursts up to 30 A (charge/discharge rate > 8C). Multirotor drones use 3-12 cells in series (11.1-44.4 V) at very high C-rates with ~80 Wh/kg lithium-polymer pouches that trade cycle life for power density.
  • Medical implants. Pacemakers and implantable cardioverter-defibrillators use lithium primary cells (Li/I2, Li/MnO2, Li/CFx) for 5-10 year operation; rechargeable Li-ion variants are now appearing for neurostimulators (e.g., Boston Scientific Vercise, Abbott Proclaim) where wireless charging extends device life from 5 to 25 years.

Frequently asked questions

What happens chemically when a Li-ion cell charges and discharges?

On charge, Li+ deintercalates from the cathode lattice (e.g., LiCoO2 → Li(1-x)CoO2 + xLi+ + xe-) and migrates through the electrolyte to the graphite anode, where it intercalates between graphene layers (xLi+ + xe- + 6C → LixC6, full charge x=1, giving LiC6). The electron flows through the external circuit as charging current. On discharge, Li+ moves the opposite direction: extracted from LiC6, ferried back through the electrolyte, and reinserted into the cathode. Because Li+ ions are physically rocking between two host structures rather than being plated as metallic Li, the chemistry is also called a 'rocking-chair' battery. Reversibility over 1000-3000 cycles is achievable with modern formulations.

What are NMC, NCA, and LFP and how do they differ?

All three are layered or olivine transition-metal oxide cathodes that replaced or augmented the original LiCoO2. NMC (LiNixMnyCozO2 with x+y+z=1) blends nickel for high capacity, manganese for structural stability, and cobalt for rate capability. NMC811 (Ni0.8Mn0.1Co0.1O2) is the high-nickel state of the art at ~200 mAh/g. NCA (LiNi0.8Co0.15Al0.05O2) used by Tesla 18650 cells substitutes aluminum for manganese, gaining ~20% energy density at the cost of thermal stability. LFP (LiFePO4, lithium iron phosphate) trades 30% energy density for cobalt-free chemistry, longer cycle life (3000-7000+ cycles), and exceptional thermal stability — it dominates Chinese EVs and stationary storage. Voltage windows: NMC and NCA 3.6-4.2 V, LFP 3.0-3.65 V.

Why is the nominal cell voltage 3.7 V?

The cell voltage equals the difference between cathode and anode redox potentials versus Li/Li+. Graphite intercalation occurs at about 0.1-0.2 V vs Li/Li+ (so the anode sits ~0.1 V above metallic Li); LiCoO2 deintercalation occurs at about 3.9 V vs Li/Li+ over most of its stoichiometric range. The difference is roughly 3.7-3.8 V, which is the nominal voltage stamped on the cell. Maximum charging voltage of 4.2 V corresponds to the deintercalation cutoff that keeps the LiCoO2 lattice from collapsing; minimum discharge voltage of 2.5-3.0 V protects the graphite from over-extraction. The 3.7 V is high enough to power most consumer electronics directly without step-up conversion, which is one reason Li-ion displaced Ni-MH (1.2 V) in laptops by 1995.

What is the role of the SEI layer?

On the first charge, the electrolyte (typically LiPF6 in EC/DMC/DEC carbonate solvents) reduces irreversibly at the graphite surface to form a Solid Electrolyte Interphase (SEI) — a 10-50 nm thick layer of Li2CO3, lithium alkyl carbonates, LiF, and polymeric byproducts. The SEI is electronically insulating but ionically conductive (Li+ passes through), and it protects the graphite from further electrolyte reduction during normal operation. SEI formation consumes 5-15% of the cell's first-cycle Li capacity (the so-called 'formation loss'), which is why factories perform a slow first-charge cycle. When the SEI cracks during cycling — from volume changes, fast charging, or low-temperature plating — fresh electrolyte reduces and the SEI grows, irreversibly consuming Li and capacity. SEI degradation is the dominant capacity-fade mechanism in modern Li-ion.

Why do Li-ion batteries catch fire?

Thermal runaway happens when internal heat generation outpaces dissipation, raising temperature into a self-sustaining decomposition cascade. The chain: SEI decomposes around 80-120 C, releasing flammable gases. Above 130 C the separator (polyethylene-polypropylene, melting point ~135-165 C) softens and shorts. The cathode (especially layered NMC/NCA) releases oxygen above 200 C; the electrolyte ignites; and the cell vents at 800-1000+ C. Triggers include internal short circuits from manufacturing defects (a metal flake, the Galaxy Note 7 case), overcharging, mechanical crush, or external heat. LFP cathodes are far more thermally stable (oxygen release only above 300 C) and rarely propagate runaway. Cell-level safety design — current interrupt devices, vents, ceramic-coated separators — and pack-level thermal management mitigate but cannot eliminate the risk.

What is the path to solid-state batteries?

A solid-state battery replaces the liquid carbonate electrolyte with a solid lithium-ion conductor (sulfide glasses such as Li10GeP2S12, oxide garnets such as Li7La3Zr2O12, or polymers). The promised wins: a metallic Li anode (dendrites suppressed by mechanical confinement) for ~3x volumetric capacity, no flammable solvent, and operating ranges from -40 to +120 C. Toyota, Solid Power, QuantumScape, and Samsung SDI all have running prototypes; the engineering challenges that have delayed mass production for two decades are interfacial resistance between electrolyte and electrodes, manufacturing scale-up of thin solid sheets, and dendrite penetration of brittle ceramics. Honda announced a pilot line in 2025; commercial EV cells are widely projected for 2027-2030 if the roadmap holds.