EV battery cell chemistry: electrode materials, electrolytes and separators

LFP, NMC and NCA name the positive-electrode materials used in many EV batteries. Pairing those materials with graphite, silicon or another negative electrode, and choosing a compatible electrolyte, shapes the cell's energy storage, charging behaviour and durability.

Battery Basics explains how a cell stores energy and moves ions. Here, the focus is the materials and the way they are assembled inside the cell.

Positive-electrode materials

The positive electrode, conventionally called the cathode, affects both the amount of charge that can be stored and the electrode potential. Cell voltage is the difference between the potentials of the two electrodes.

NMC, NCA and NCMA

NMC means lithium nickel manganese cobalt oxide. NCM is another name for the same family of layered oxides.

In NMC notation, numbers such as 111, 622 and 811 specify the approximate molar ratio of nickel, manganese and cobalt. NMC 811 has a Ni:Mn:Co ratio of 8:1:1. The digits follow the element order: NMC 532 and NCM 523 both describe Ni:Mn:Co in a 5:3:2 ratio. These are ratios of the transition metals in the cathode material, rather than percentages of the complete cell. iScience: NCM523 composition in electrode manufacturing research (2020)

Increasing nickel content can raise the material's charge-storage capacity and reduce the proportion of cobalt. The accompanying challenge is maintaining stability at high states of charge. Particle cracking and reactions with the electrolyte can accelerate ageing, so nickel-rich materials need careful control of particle structure and surface coatings. Nature Communications: silicon-containing anodes and complete-cell design (2021)

NCA is lithium nickel cobalt aluminium oxide. It also uses a layered structure, with aluminium helping to improve thermal stability. Nature Communications: silicon-containing anodes and complete-cell design (2021) LG Energy Solution: NCA cathode materials (2023) NCMA adds aluminium to a nickel-cobalt-manganese oxide system. LG Energy Solution says aluminium contributes to power output and safety, while using more nickel and less cobalt reduces material cost. LG Energy Solution: NCMA cathode materials (2023)

LFP

LFP is lithium iron phosphate, an olivine-structured material containing neither nickel nor cobalt. Its phosphate framework provides strong thermal and chemical stability, making it attractive for durable cells with reduced reliance on those metals.

LFP operates at a lower voltage and stores less charge per gram than high-capacity nickel-rich cathodes, limiting its energy density. Its lower electrode density also reduces how much energy fits into a given volume. University of Warwick / Energy & Fuels: practical LMFP cathode trade-offs (2025)

LFP has another distinctive property: a broad, relatively flat voltage plateau. As lithium moves in and out, lithium-rich and lithium-poor phases coexist over much of the material's operating range. The voltage changes little across that region, making voltage alone a weak indicator of state of charge. Battery-management systems therefore need to combine measurements with a model of the cell. npj Computational Materials: LFP/graphite voltage behaviour and modelling (2024)

Battery Management System explains state-of-charge estimation; Battery Charging covers vehicle charging limits.

LMFP

LMFP is lithium manganese iron phosphate. Replacing some of the iron in LFP with manganese introduces a higher-voltage part of the discharge curve, creating a route to more energy from a phosphate cathode.

The gain brings additional engineering work. Poor electronic and ionic conductivity can limit power, while manganese dissolution can shorten life. Particle size, carbon coatings and the manganese-to-iron ratio affect how much of the voltage advantage survives in a practical electrode.

A 2025 University of Warwick study tested commercially supplied LMFP materials against LFP in graphite-based laboratory coin cells. The material with an 80:20 manganese-to-iron ratio initially delivered 18% more energy per mass of cathode active material than LFP at C/20, a low discharge rate. Poorer rate performance and faster fading erased that advantage during a subsequent 100-cycle test at C/3. These measurements describe the tested materials and coin cells, rather than the energy density or lifetime of an automotive cell. University of Warwick / Energy & Fuels: practical LMFP cathode trade-offs (2025)

LMO and blended cathodes

LMO is lithium manganese oxide, usually with a spinel crystal structure. Its three-dimensional lithium diffusion paths support high power, but manganese dissolution and capacity loss can limit durability. ChemSusChem: LMO power characteristics and blended cathodes (2020) Nature Communications: manganese dissolution and LMO capacity loss (2019)

Manufacturers can combine materials within one electrode. LMO/NMC blends are one example; LMFP/NMC blends are another. The proportions change the voltage profile, electrode density and balance between power and energy. A blend is physically different from a pack containing separate cells of different chemistries. University of Warwick / Energy & Fuels: practical LMFP cathode trade-offs (2025) ChemSusChem: LMO power characteristics and blended cathodes (2020)

The following comparison concerns positive-electrode materials paired with a suitable negative electrode. It describes tendencies rather than fixed cell specifications.

Pack structure changes the relationship between cell and pack energy density. Those effects, including cell-to-pack and structural integration, are covered in Battery Pack & Configuration.

Negative-electrode materials

The negative electrode is conventionally called the anode. Graphite stores lithium between its carbon layers, silicon forms lithium-silicon alloys, and lithium-metal electrodes store it as metal. Sodium-ion cells use a different material system, discussed below.

Graphite

Graphite contains layers of carbon that can accommodate lithium between them. This storage process, called intercalation, is reversible and is the basis of conventional graphite-anode lithium-ion cells.

Electrodes can use natural graphite, synthetic graphite or blends. Particle size, shape and surface treatment influence how quickly lithium can enter the material and how much electrolyte reacts at its surface.

During charging, metallic lithium can deposit on graphite instead of entering its structure. This is lithium plating. High charging current and low temperature can favour plating by making ion insertion too slow for the imposed current. Argonne / DOE review presentation: Anode Structures that Enhance Fast Charge (2020) Some deposits can return to cycling; others become electrically isolated or react with electrolyte, reducing the available lithium. If deposits grow through the separator, they can create an internal short circuit. Nature Communications: lithium plating, inactive lithium and short-circuit mechanisms (2022)

The graphite limitation also applies to graphite/LFP cells. A stable positive electrode does not remove a charging constraint on the negative electrode. Battery Thermal Management explains how heating and cooling help control these conditions.

Silicon-containing electrodes

Silicon stores lithium by forming lithium-silicon alloys and can store much more charge per gram than graphite. It also changes size substantially: full lithiation can increase the silicon active material's volume by roughly 300%. That figure describes the silicon itself, rather than the swelling of a complete electrode or cell. Nature Communications: silicon-containing anodes and complete-cell design (2021)

Expansion and contraction can break particles and their electrical connections. They can also damage the solid-electrolyte interphase, or SEI: the protective reaction layer formed on the negative electrode. Rebuilding that layer consumes electrolyte and lithium that would otherwise remain available for cycling.

Graphite-silicon composites retain graphite as a stable framework while adding silicon or silicon-based compounds, including silicon oxides. Engineered particles and flexible binders help accommodate movement; electrolyte additives help form a more resilient interface.

A large increase in anode-material capacity produces a smaller increase in finished-cell energy density. The positive electrode, electrolyte, separator, collectors and enclosure still contribute mass, and the two electrodes must have compatible capacities. Nature Communications: silicon-containing anodes and complete-cell design (2021)

Lithium titanate

Lithium titanate, or LTO, is an alternative negative-electrode material. It operates at a higher potential than graphite, giving more margin against lithium plating and supporting high power and frequent cycling.

Its crystal structure changes very little in volume as lithium enters and leaves, helping it withstand repeated cycling. LTO also stores less charge per gram than graphite. Combined with the lower cell voltage produced by its higher anode potential, this reduces energy density. Lancaster University: LTO structure and capacity, author manuscript

LTO therefore suits applications where charging frequency and service life matter more than maximum energy in a small package. The US Department of Energy identifies LTO with high-power and high-cycle-life applications; Toshiba's LTO-based SCiB cells provide a commercial example. US Department of Energy: lithium-ion technology assessment (2023) Toshiba: LTO-based SCiB cell characteristics

Lithium metal

A lithium-metal electrode replaces graphite or silicon as the negative-electrode active material, offering a route to higher cell energy density. During charging, lithium deposits onto the negative electrode; during discharge, it is stripped away.

Maintaining uniform deposition and removal is difficult. Reactions with electrolyte can leave lithium electrically isolated, consume electrolyte and damage the separator. These processes connect the energy-density opportunity directly to durability and safety challenges. US Department of Energy: lithium-metal cell failure mechanisms (2023)

Lithium metal describes the negative electrode. Solid-state describes the electrolyte system. They can be used together, but neither term requires the other.

Sodium-ion material systems

Sodium-ion cells use sodium-containing electrode materials and electrolytes. Hard carbon is a leading negative-electrode material because conventional graphite stores little sodium with the carbonate-based electrolytes commonly used in these cells. Nature Communications: sodium storage in hard carbon (2024)

Hard carbon

Hard carbon has disordered carbon layers, defects and pores. Sodium storage involves several sites and processes, including storage between layers, at defects and within nanopores. Their relative contributions depend on the carbon's structure and remain an active research topic.

The precursor and heat treatment influence that structure. Increasing accessible surface area can help transport, but can also expose more surface to electrolyte reactions. Sodium consumed in forming the SEI is unavailable for later reversible charge storage, reducing first-cycle efficiency: the share of the first charge that can be recovered on discharge. Nature Communications: sodium storage in hard carbon (2024)

Hard carbon's capacity and packing density also constrain finished-cell energy density. A 2026 research review identifies both as continuing challenges for higher-energy sodium-ion cells. Nature Reviews Materials: sodium-ion anode capacity and density constraints (2026)

Positive-electrode families

Three major families offer different approaches. Faraday Institution / Rho Motion: sodium-ion material families, printed pp. 43–44 (2023)

• Layered oxides use combinations of metals in layered structures. The metal combination and crystal structure affect capacity and stability during cycling.
• Polyanionic materials use frameworks such as phosphates and sulphates. Their three-dimensional frameworks can favour durability, with energy density and conductivity depending on the specific compound.
• Prussian blue analogues have open frameworks that accommodate sodium ions. Water content and vacancies in the framework affect their structure and electrochemical performance. Chemistry – A European Journal: water, vacancies and sodium content in Prussian blue analogues (2025)

Sodium-ion designs can avoid lithium and, with suitable cathodes, nickel and cobalt. Lower-cost raw materials alone do not establish a lower cell price: the amount of energy stored, manufacturing yield and production scale also matter. Current product claims and commercial status are covered in Latest in Battery Technology.

How electrode construction changes performance

A conventional electrode is a porous coating on a metal current collector. Its active material stores charge; conductive additives connect the particles electronically; and a binder holds the coating together and attaches it to the foil. Electrolyte fills the pores so that ions can reach the active material.

Loading more active material onto each unit of foil area can increase energy density by spreading the mass of the foil and separator over more stored energy. At a given coating composition and porosity, this means a thicker electrode and a longer path for ions. A thinner electrode shortens that path, at the cost of more foil and separator per unit of stored energy.

Thick coatings are also harder to dry uniformly. Cracks or an uneven distribution of binder can undermine their performance, so the coating must remain connected, permeable and mechanically intact. LBNL/DOE: high-energy-density electrode construction (2019)

Porosity describes the fraction of the coating occupied by voids. More pore space can improve electrolyte access, while reducing the amount of active material that fits into a given volume. The connections between pores matter too. Tortuosity describes how indirect the transport paths are: straighter, better-connected paths can improve ion transport without increasing the pore volume. In a laboratory study of thick, sintered electrodes, aligned pores retained more discharge capacity at higher cycling rates than a structure with a similar pore-volume fraction but no preferred alignment. NIST: pore alignment and transport in thick electrodes (2021)

These choices explain how two cells with the same active materials can be designed for different priorities: storing more energy in a limited space, or accepting and delivering that energy more quickly.

Electrolytes and protective interfaces

An electrolyte must carry ions while limiting unwanted reactions at both electrodes. A common lithium-ion formulation uses lithium hexafluorophosphate (LiPF₆) dissolved in a mixture of carbonate solvents, such as ethylene carbonate and ethyl methyl carbonate. Sodium-ion cells use compatible sodium salts. US Department of Energy: lithium-ion technology assessment (2023) Journal of Physics: Energy: current-collector corrosion and passivation (2021)

Additives help control the reactions at electrode surfaces. For example, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) can help form protective interfaces in cells containing silicon. Their effectiveness depends on the electrode combination and operating conditions; the additive can itself be consumed or produce unwanted reaction products. Nature Communications: silicon-containing anodes and complete-cell design (2021)

SEI, CEI and cell formation

Some electrolyte decomposes during a cell's first controlled charge and discharge cycles, known as formation. The resulting SEI on the negative electrode helps prevent continued reaction between the charged electrode and the electrolyte. It must allow ions through while restricting electron transfer to the electrolyte.

A related surface layer on the positive electrode is called the cathode-electrolyte interphase, or CEI. Its stability becomes particularly important at high positive-electrode potentials.

A stable interface limits further electrolyte reactions. A layer that repeatedly cracks, dissolves or grows can increase resistance and reduce available capacity. This is why surface coatings, electrolyte additives and the formation process are integral to the cell's performance. Nature Communications: silicon-containing anodes and complete-cell design (2021) The longer-term consequences are explained in Battery Degradation.

Liquid, gel and solid electrolytes

Liquid electrolyte wets a porous electrode and reaches the surfaces of its particles. Gel systems retain liquid within a supporting network. Solid electrolytes use ion-conducting materials such as polymers or inorganic compounds, which require suitable contact with the electrodes.

Solid-state cells can retain familiar cathode materials such as NMC. Their negative electrode may use lithium metal or an alternative active material such as silicon. Nature Communications: silicon-containing anodes and complete-cell design (2021)

Replacing liquid with solid electrolyte changes the interfaces. Reaction layers can raise resistance, and expansion and contraction of the active material can break contact with a rigid solid electrolyte, interrupting ion transport. Maintaining low-resistance contact is therefore a central engineering task. DOE / University of Maryland: solid-electrolyte interface engineering (2021) Nature Communications: volume change and contact loss in solid-state cells (2024)

Terms such as semi-solid and quasi-solid can refer to designs retaining liquid or gel. Identifying the electrolyte and both electrodes gives a clearer description than the solid-state label alone.

Separators and current collectors

Separators

In a liquid-electrolyte cell, a porous separator keeps the electrodes apart while allowing ions to pass through electrolyte-filled pores. Common materials include polyethylene and polypropylene; ceramic coatings can improve heat resistance.

Some separators provide thermal shutdown: pores close as the polymer softens, restricting ion transport and current. Protection has limits. Further heating can destroy the separating layer, and puncture or other damage can create a short circuit before shutdown helps. Oak Ridge National Laboratory: separator materials and shutdown limits, section 3.1 (2023)

Separator thickness and pore structure also affect resistance and the space left for active material. Cell-level protection and pack-level measures are explained in Battery Safety.

Current collectors

Metal foils carry electrons between the electrode coatings and the cell terminals. Conventional graphite-anode lithium-ion cells usually use copper on the negative side and aluminium on the positive side.

The choice follows the electrode's operating potential and its interaction with the electrolyte. Aluminium can alloy with lithium at the low potential of a charged graphite electrode, so copper is used there. At positive-electrode potentials, copper can oxidise and dissolve. Aluminium is protected by a surface film, including a fluoride-rich layer in conventional LiPF₆ electrolytes. Journal of Physics: Energy: current-collector corrosion and passivation (2021)

LTO operates at a higher potential than graphite and can use aluminium on the negative side as well. US Department of Energy: lithium-ion technology assessment (2023) Sodium-ion cells can also use aluminium collectors on both sides because sodium does not electrochemically alloy with aluminium at room temperature. Small: lithium/sodium chemistry and current-collector choices (2026)

Reducing collector thickness saves inactive mass, but the foil must remain strong enough for manufacturing and service. The arrangement of these coated foils, separators and terminals is covered in Cell Formats.