Preventing Metal-Ion Cross-Talk via Gel Polymer Electrolyte for Li-Ion Batteries at High-Voltage
Authors: Kim Daehyun , Jeong Jihong , Song Hyun-Kon 
Published: December 2024

The pursuit of high energy density batteries drove the utilization of a high voltage operational range using nickel-rich layered oxides, represented as LiNixCoyMnzO2 (NCM; x + y + z = 1, x ≥ 0.6). It is a straightforward method in that it can increase energy density by simultaneously increasing capacity and average discharge voltage without changing electrodes and electrolytes. However, during high-voltage operation (> 4.5 V versus Li/Li+), Ni-rich NCM undergoes extensive delithiation, leading to the formation of high-valence Ni4+ ions.

These high-valence nickel ions are susceptible to decomposing the electrolyte and reducing into low-valence Ni cation (Ni3+ and Ni2+). As a result of the aforementioned side reaction, the cross-talk of dissolved Ni2+ toward the anode can be accelerated. The deposition of Ni2+ at the anode surface suppresses the intercalation of Li+ into the anode layer structure. Additionally, the migration of Ni2+ to adjacent lithium vacancy sites in cathode, leads to the formation of an electrochemically inactive rock-salt phase. Due to these reasons, significant capacity degradation occurred as the cycles progressed, posing challenges despite the high initial capacity. Additionally, in the charged state, the release of oxygen due to structural collapse increases the risk of fire.

In this study, we aimed to enhance the charging performance of NCM811 up to 4.6 V, specifically within the full cell range of 4.55 V. To mitigate the problem of high voltage operation, we introduced a gel polymer electrolyte (GPE) made by PVA-CN which is a cross-linked polymer. Only 2 wt. % of PVA-CN is needed in carbonate-based electrolytes for gelation. We attached N, N-Disuccinimidyl carbonate moiety (SC) to the hydroxyl groups of PVA-CN for (1) anchoring and (2) chelating transition metal ions to inhibit the side reactions of high-valence metal ions or dissolved metal ions from the cathode. FT-IR and NMR analysis confirmed the 6% of SC attached to PVA-CN (SC-PVA-CN). Even with this small addition, it showed superior performance compared to the general carbonate-based liquid electrolytes.

Highly oxidized nickel ions tend to take electrons away from the electrolyte, undergoing reduction to form Ni2+ ions. Inclusion of anchoring polymers in the electrolytes maintained the transition metals in a stable state during charging processes, inhibiting side reactions with the electrolyte. In the case of liquid electrolytes, the ratio of Ni2+ and Ni3+ was dominant at the charged state after formation cycles. However, the ratio of Ni4+ was increased when the SC-PVA-CN GPE was present in the electrolyte. This is because bidentate coordination to Ni4+ created an environment similar to Ni2+ at the charged state. High ratio of Ni4+ at the charged state is beneficial in maintaining the structure under high voltage operating conditions by reducing the leakage of nickel from the cathode. Transition metal ions better maintain their place and act as a pillar within the cathode structure during the process of repeated charge and discharge.

Maintaining the cathode structure also suppresses the formation of particle cracks. By reducing the amount of electrolyte penetrating between particle cracks, interface side reactions were also suppressed, decreasing phase changes inside the material, and maintaining lithium-ion transmission path.

It is known that one transition metal ion takes up two lithium ions for intercalation to the anode. Some Ni2+ ions generated from the cathode because the surface is not completely covered with GPE can also be prevented from moving to the anode. After Ni2+ dissolution, they were chelated by functional groups of SC-PVA-CN in the bulk electrolyte region and reduced the amount of transition metal ions deposited on the anode. Therefore, more lithium ions can reversibly de/intercalate to the anode, mitigating irreversible capacity loss, which greatly contributes to maintaining capacity at high voltage operation.

Additionally, the non-flammability was enhanced by utilizing SC-PVA-CN GPE. When igniting the liquid electrolyte and the SC-PVA-CN GPE using a torch, the liquid electrolyte ignited for 41 s g-1, while the SC-PVA-CN GPE did not ignite. This GPE formed nitrile-nitrile conjugation bonding which contained high bonding energy. In addition, interaction between solvent mitigated the volatility of liquid parts.

In short, by utilizing SC-PVA-CN GPE, it showed better performance in the high voltage operation due to anchoring and chelating. Additionally, it showed non-flammability, enabling both high energy density and better safety.

Figure 1