The emergence of lithium-ion batteries is a leap in the history of lithium secondary batteries, which makes lithium batteries formally enter the field of large-capacity energy storage applications. Lithium-ion battery cathode materials not only participate in electrochemical reactions as electrode materials, but also serve as lithium ion sources. Therefore, the positive active material of lithium-ion battery must meet the following standards:
1) Higher than energy;
2) The electrode potential is high;
3) The reversibility of the charge and discharge reaction is good;
4) Good chemical stability in electrolyte, low solubility (small self-discharge);
5) It has high electronic conductivity;
6) Abundant resources and low prices.
In addition, in a lithium secondary battery, since the positive electrode provides lithium required for reciprocating insertion and extraction between the positive and negative electrode intercalation compounds, the positive electrode active material must be a lithium-containing compound. At present, research on cathode materials for lithium-ion batteries is mainly concentrated in the following types of materials:
1) One type is the transition metal oxide LiMO2 (M=Ni, Co, Mn, Fe, V, etc.) with a layered structure and its derivatives;
2) One type is a substance with a three-dimensional network structure, mainly some spinel-type compounds LixM2O4 (M=Mn, Ti, V);
3) One type is olivine-type lithium intercalation compounds, such as LiFePO4.
The four main lithium-ion battery cathode materials and their main properties are shown in Table 1. We can see that: from the comprehensive consideration of technical performance and cost, lithium iron phosphate (LiFePO4) battery has the most competitive advantage.
| Material | LiCoO2 | L.iNiO2 | LiMn2O4 | LiFePO4 |
| Theoretical capacity/(Ah/kg) | 275 | 274 | 148 | 170 |
| Actual capacity/(Ah/kg) | 160 | 220 | 110 | 160 |
| Bulk density/(kg/L) | 5.05 | 4.8 | 4.2 | 3.7 |
| Volume capacity density/(Ah/L) | 808 | 1 056 | 462 | 592 |
| Discharge curve shape | Straight | tilt | Straight | Straight |
| safety | generally | Difference | good | very good |
| cost | high | Centered | Low | Low |
Lithium-ion batteries have the characteristics of high efficiency and high energy density, and have the advantages of stable discharge voltage, wide operating temperature range, low self-discharge rate, long storage life, no memory effect, and no pollution. However, in the past, lithium-ion batteries had certain problems in large-scale manufacturing, and the special packaging requirements for overcharge control were high and expensive, so they were not widely used. But recently, the large-capacity integration technology of lithium-ion batteries (especially LiFePO4) has made a major breakthrough, opening a new chapter in the application of lithium-ion batteries’ large-capacity energy storage technology.
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In 2010, the successful trial operation of the 100kW-LiFePO4 energy storage demonstration project constructed by the Shanghai Electric Power Company in China opened the prelude to the application of large-capacity lithium battery energy storage.
In 2011, China Southern Power Grid Corporation’s megawatt (total capacity 10MW) lithium battery energy storage station was successfully connected to the grid.