What Is Characteristics of Lithium-ion Batteries?
Capacity and Electromotive Force of Lithium-ion Battery Materials
During the charge-discharge reaction of lithium-ion batteries, only the active materials of the positive and negative electrodes undergo lithium-ion intercalation/deintercalation reactions, while the electrolyte and other materials are not consumed. Therefore, the potential at which the positive and negative electrode materials reversibly intercalate/deintercalate lithium ions determines the open-circuit voltage of the battery, and the amount of lithium ions intercalating/deintercalating determines the capacity of the active material. Many global lithium-ion battery manufacturers and lithium-ion battery suppliers rely on these material characteristics to achieve stable mass production and consistent product performance.
For the negative electrode, the reaction occurs according to Equation (1.2). Per mole of carbon (12g), a maximum of 1/6 mol of lithium ions can be intercalated. Therefore, the theoretical specific capacity of carbon negative electrode material is
1/6(mol)×96485(Faraday constant,C/mol)/12(g)=3400C/g=372(mA·h/g) (1.5)
In daily use, considering lithium loss due to adsorption and the formation of the solid electrolyte interphase (SEI) film, the actual achievable specific capacity of carbon materials is 300–345 mA·h/g. Leading lithium-ion battery pack suppliers achieve this level through optimized graphite formulation and precise coating processes.
For the positive electrode material, its capacity depends on the amount of lithium ions that can be extracted/inserted. Taking LiCoO₂ as an example, up to 1 mol of lithium ions per mole of LiCoO₂ can participate in the reaction. Therefore, the theoretical specific capacity of LiCoO₂ (relative molecular mass 97.86) is
1(mol)×96485(C/mol)/97.86(g)=985.95C/g=273.9(mA·h/g) (1.6)
In practice, to maintain the crystal stability of LiCoO₂ material, generally only 30%–60% of the lithium ions participate in the reaction. Therefore, the actual specific capacity of LiCoO₂ material is 137–164 mA·h/g. Major lithium-ion battery OEM manufacturers control the depth of charge and discharge through advanced BMS to maximize cycle life while ensuring safety.
For lithium iron phosphate, 1 mol of lithium ions per mole of lithium iron phosphate can fully participate in the reaction. Therefore, the theoretical and actual specific capacity of lithium iron phosphate material (relative molecular mass 157.8) is
1(mol)×96485(C/mol)/157.8(g)=611.44C/g=169.8(mA·h/g) (1.7)
In nature, the standard redox potential of Li/Li⁺ is the lowest, reaching -3.04 V (vs. standard hydrogen electrode). For carbon negative electrode materials, the potential of lithium-ion extraction and insertion is near the Li/Li⁺ equilibrium potential. According to electrochemical theory, at room temperature, the electrode potential E of the carbon negative electrode is
E = E° + 0.02567 · ln[C(Li⁺)/C(Li,C₆)] (1.8)
where
E° - standard electrode potential;
C(Li⁺) - concentration of lithium ions in the electrolyte solution;
C(Li,C₆) - concentration of lithium ions in the negative electrode carbon.
When the lithium ion concentration in the solution and in the negative electrode carbon are close, the electrode potential of the negative electrode equals the standard reduction potential E°. Generally, the lithium ion concentration in the electrolyte is fixed, so changes in lithium ion concentration in the negative electrode carbon will cause changes in the negative electrode potential. There is currently no universal method to calculate the precise equilibrium potential of Li/C₆ with varying x values. It is generally determined experimentally. Experiments show that the delithiation potential of graphite-based materials generally varies between 0–0.4 V (vs. Li/Li⁺), making them relatively suitable negative electrode materials for applications. Figure 1.2 shows the typical charge-discharge characteristic curve of a graphite negative electrode.
For LiCoO₂ positive electrode material, the lithium intercalation/deintercalation process is a single-phase reaction. As the lithium ion concentration in the positive electrode material changes, the potential of the positive electrode also changes. Considering the lithium ion concentration in the electrolyte is 1 mol/L, for the reaction in Equation (1.1), the positive electrode potential E is
E = E° + 0.02567 · ln[C(Li⁺,CoO₂)/C(LiCoO₂)] (1.9)
where
E° - standard electrode potential;
C(LiCoO₂) - concentration of LiCoO₂ in the positive electrode material;
C(Li⁺,CoO₂) - concentration of Li⁺ and CoO₂ in the positive electrode material;
As lithium ions are extracted, the positive electrode potential shows a downward trend.
The charge-discharge process of lithium iron phosphate material is the conversion from lithium iron phosphate to iron phosphate after delithiation.
The reaction at the lithium iron phosphate electrode is
LiFePO₄ ↔ FePO₄ + Li⁺ + e⁻ (1.10)
Its lithium-ion intercalation/deintercalation process is a two-phase reaction. Therefore, changes in lithium ion concentration in the positive electrode material do not affect the potential change of the positive electrode. Its equilibrium potential is
E = E° + 0.02567 · ln[C(FePO₄)/C(LiFePO₄)] (1.11)
The concentration of pure solids is 1. Based on its thermodynamic parameters, the theoretical equilibrium potential is 3.4 V.
The typical charge-discharge characteristic curve of lithium iron phosphate material is shown in Figure 1.3.
Performance Characteristics of Lithium-ion Batteries
Compared with other batteries, lithium-ion batteries have the following characteristics that are widely recognized by lithium-ion battery distributors and industrial clients:
High energy density. The energy density of lithium-ion batteries reaches 100 W·h/kg and 200 W·h/L or more. Recent ternary cathode lithium-ion batteries have achieved a mass specific energy of 200 W·h/kg. Using high-nickel silicon-based anode materials and lithium-rich cathode materials, the mass specific energy is expected to reach 400 W·h/kg and the volumetric energy density 900 W·h/L, far exceeding traditional batteries. Therefore, lithium-ion batteries are widely used in portable electronic products and electric vehicles.
High open-circuit voltage. Due to the use of non-aqueous organic solvents, the single-cell voltage reaches 3.6–3.8 V, which is 2–3 times that of nickel-metal hydride or nickel-cadmium batteries. Effectively utilizing high-voltage cathode materials can increase the operating voltage of a single cell to 4.5–5 V, which is one of the important reasons for the high energy density of lithium-ion batteries.
Capable of high-rate charge and discharge. For example, all-solid-state lithium-ion batteries using polymer electrolytes can achieve discharge rates above 10C with good safety; lithium-ion batteries using lithium iron phosphate as the cathode can achieve 100C discharge.
Low self-discharge rate. At room temperature, the monthly self-discharge rate of lithium-ion batteries is generally less than 10%, lower than nickel-metal hydride batteries (15%) and half that of nickel-cadmium batteries. The self-discharge rate of lithium iron phosphate batteries is generally less than 3%.
Environmentally friendly, containing no lead, cadmium, mercury, or other harmful substances, and does not pollute the environment.
No memory effect. Memory effect refers to the phenomenon where the battery capacity decreases when recharged before being fully discharged or used before being fully charged (memory effect is not capacity decay). Lithium-ion batteries have no memory effect.
Good safety. Lithium-ion batteries generally use carbon materials as the negative electrode, which has an electrode potential close to that of metallic lithium. Lithium ions can reversibly intercalate and deintercalate in carbon, greatly reducing the probability of lithium metal deposition and significantly improving battery safety. In recent years, flame-retardant additives, flame-retardant separators, PTC (positive temperature coefficient) devices, explosion-proof valves, battery management systems, and other technologies have ensured extremely high safety of lithium-ion batteries.
Long cycle life. The cycle life of lithium-ion batteries is generally more than 500 cycles. The cycle life of lithium iron phosphate batteries is generally 2000–3000 cycles. When matched with anode material systems with high cycle capability (such as lithium titanate), more than 10,000 cycles can be achieved. This makes lithium iron phosphate batteries the best choice for energy storage battery systems and large-scale ESS projects.
