High nickel content LiNi x Co y Mn 1−x−y O 2 (NCM) cathode materials have been attracting increasing attention owing to their significant advantages, but in practical application, because of their poor storage performance their production and transportation cost a lot. The gap between polycrystalline particles can very easily become the site where impurities are first generated; thus, whether the single crystal structure will affect the storage properties of high nickel content NCM materials is worth studying. In this work, we take two typical high nickel content ternary materials, LiNi 0 . 8 Co 0 . 1 Mn 0 . 1 O 2 (NCM811) and Li-Ni 0.6 Co 0.2 Mn 0.2 O 2 (NCM622), as samples to study the differences in properties of single-crystal and polycrystalline materials after storage. Through comparative research, under the same storage conditions, the single-crystal structure materials have significantly less impurities formed on the surface, the structural stability of materials is obviously better, and they can also exhibit superior electrochemical performance after storage. Particularly for the NCM811 materials with a higher nickel amount, the specific capacity of polycrystalline NCM811 materials basically comes nearly to zero after storage, but the single-crystal structure NCM811 materials can still perform a relatively stable cycle with a certain capacity. It can be concluded from the work that for high nickel content NCM cathode materials, the single-crystal structure can greatly improve its storage performance.
High‐Ni‐content LiNixCoyMn1−x−yO2 is regarded as a feasible cathode material to meet the urgent requirement for high energy density batteries. However, such cathode has a poor safety performance because of reactive oxygen releasing at elevated temperatures. In pursuit of high‐safety lithium‐ion batteries, a heatproof–fireproof bifunctional separator is designed in this study by coating ammonium polyphosphate (APP) particles on a ceramic‐coated separator modified with phenol‐formaldehyde resin (CCS@PFR). The CCS@PFR separator acts as a thermal‐supporting layer to inhibit the shrinkage of the separator at elevated temperatures, whereas the APP‐coated layer functions as a fireproof layer, forming a dense polyphosphoric acid (PPA) layer above 300 °C. The PPA layer not only isolates the combustibles from the highly reactive oxygen released from the cathodes but also converts violent combustion reactions into mild stepwise exothermic reactions by carbonizing the combustibles in the batteries. Enabled with such a heatproof–fireproof bifunctional separator, LiNi0.8Co0.1Mn0.1O2|SiOx−Gr full cells are constructed and these exhibit an excellent safety performance by not catching fire during a 30 s combustion test and surviving the 10 min high‐temperature test above 300 °C. Additionally, an adiabatic rate calorimeter and nail penetration test are conducted with 3 Ah LiNi0.8Co0.1Mn0.1O2|SiOx−Gr pouch cells to further verify the safety performance.
The degradation mechanism of the stored LiNiCoMnO (NCM523) electrode has been systematically investigated by combining physical and electrochemical tests. After stored at 55 °C and 80% relative humidity for 4 weeks, the NCM523 materials are coated with a layer of impurities containing adsorbed species, LiCO and LiOH, resulting in both the weight gains of the materials and the electrochemical performance deterioration of the electrode. The impurities generated in air will react with the electrolyte and instantly turn into Li POF and other species containing the decomposition products of electrolyte when the stored NCM523 materials are soaked into the electrolyte, causing the charge potential plateau and the impedance to ascend. For the stored NCM523 electrodes, the huge and changeable impedance deteriorates the discharge capacity in the first 10 cycles and the discharge capacity will slowly recover and stabilize within 10 cycles when charging/discharging in 0.1 or 0.2 C. The thermal stability of the stored NCM523 materials get slightly better due to the relatively lower delithiated state after charged to 4.3 V.
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