With the rapid popularization of electric vehicles and energy storage devices, the market's demand for power batteries is not only about energy density and safety, but also about fast charging capability and long lifespan. Traditional liquid electrolyte lithium-ion batteries often encounter safety hazards and increased capacity degradation during rapid charging, while all solid state batteries (ASSBs) have attracted high industry attention due to their potential advantages in safety and thermal stability of solid-state electrolytes.
However, achieving fast charging of ASSB at high energy density is not an easy task. Conventional solid electrolytes face limited ion diffusion, increased electrode electrolyte interface impedance, and structural degradation of composite electrodes under high rate conditions during high-speed charging, which lead to capacity degradation and poor cycle life. Previous studies have focused on improving ion conductivity, optimizing interfaces, and designing electrodes. However, achieving fast charging in practical scale and high areal loading electrodes while maintaining long cycle life remains a major challenge.
This study focuses on the combination system of NCM (LiNixMnyCozO2) positive electrode and Li6PS5Cl solid electrolyte. Through fine electrode engineering and interface optimization, it attempts to achieve high-speed (such as 15mA/cm2) charging under thick electrodes and high load conditions, while maintaining the stability of the battery for thousands of cycles. In other words, the research team aims to develop a comprehensive design guideline for all solid state batteries, enabling them to achieve fast charging with high efficiency and low loss while pursuing high energy density.
1. Experimental design and electrode construction
This study selected NCM as the positive electrode active material (CAM), Li6PS5Cl (LPSC) as the solid electrolyte, and combined with conductive and binding agents (such as CNF carbon nanofibers) and other components. The core idea is to construct a fully solid-state battery assembly scheme for 3-electrode design through a series of design criteria (i) to (ix). These design principles include:
Appropriate particle size and distribution make ion transport channels and electron conduction paths more uniform.
Optimize electrode thickness, porosity, and compaction density to achieve higher area capacity and stable interface contact.
Control the microstructure and particle ratio of the positive electrode to ensure that ion diffusion is not significantly hindered under high rate charging conditions.
The research team validated the structural stability and porosity changes of the designed electrode at different cycling times through characterization methods such as SEM, XRD, XPS, and FIB-SEM 3D reconstruction.
2. Fast charging performance test
The study first conducted rapid charging tests on a 3-electrode all solid state battery using NCM/LPSC electrolyte and Li In negative electrode configuration at 30 ℃. The charging current density gradually increased from 1mA/cm2 to 15mA/cm2 (equivalent to high rate charging of about 8C), and a lower current density (such as 1mA/cm2) was used during discharge to observe the capacity retention and cycle life under high rate charging conditions.
The results show that:
At a high charging rate of 15 mA/cm2, the battery can still achieve a high capacity of about 150/mAh/g (based on NMC active material), with an effective utilization rate of over 90%, and the charging time can be shortened to about 8 minutes. This means that fast charging from 10% SOC to 80% SOC can be achieved within 10 minutes, approaching the expectations of the electric vehicle industry for fast charging.
The battery maintains 81% capacity retention after 3000 consecutive high-speed charging cycles, with a coulombic efficiency close to 99%, demonstrating excellent long cycle stability.
This indicates that through rational microstructure design and material combination, high-speed charging and discharging with long lifespan can be achieved even at relatively low temperatures (30 ℃).
3. Microscopic mechanism and structural evolution of high rate charge and discharge
To understand such excellent cycling performance, researchers prepared cross-sectional samples using FIB-SEM and conducted 3D reconstruction analysis after 10 and 1000 cycles. As a result, it was found that:
The initial porosity of the electrode is about 3%, and after 10 cycles, the porosity slightly increases to 3.6%, and after 1000 cycles, it increases to about 6.9%. Although the porosity has increased, it is still within a controllable range. It can be seen that in high rate cycling, the microstructure of positive electrode particles undergoes certain deformation and pore increase, but it has not yet led to severe detachment or interface delamination.
There is no obvious sign of a large accumulation of surface side reaction layers. Although there may be interface stress and microcracks between the all solid electrolyte and the positive electrode particles, the interface impedance does not significantly increase with cycling through appropriate particle ratios and tight packing methods.
This structural evolution pattern indicates that in optimized composite electrodes, even if high rate charging and discharging cause certain micro pore expansion and structural stress changes, the overall conductive network is still relatively stable.
4. In depth exploration of high load and high speed conditions
To meet practical application requirements, the research team attempted to increase the thickness and loading capacity of the positive electrode active material, thereby improving the overall energy density of the battery. Result:
When the thickness of the positive electrode increases from about 70 µ m to 140 µ m and 210 µ m, high capacity utilization and cycling stability can still be achieved through charging tests at 50mA/cm2. It is worth noting that a 210 µ m thick positive electrode corresponds to a loading capacity of approximately 45 mg/cm2, which is quite significant in solid-state batteries.
Realizing fast charging on thicker electrodes indicates that the material design strategy effectively enhances the vertical diffusion ability of ions in solid electrolytes and maintains close contact between internal particles, which helps to reduce ion retention in diffusion channels.
Even in such high load electrodes, charging for 10 minutes under appropriate heating conditions (80 ℃) can achieve up to 85% utilization of active materials. The discharge also exhibits good cycling characteristics. This provides a feasible path for future large-scale electric vehicle battery applications: by using thicker electrodes and higher mass loads, higher energy output can be achieved without sacrificing fast charging performance and lifespan.
5. Electrochemical impedance and performance degradation analysis
In order to deeply analyze the mechanism of performance changes, researchers conducted alternating current impedance spectroscopy (EIS) measurements on the battery before and after cycling:
After the initial few cycles, the battery impedance slightly increased, but then stabilized over thousands of cycles. This means that if there is a micro interface adjustment initially, the basic stabilization process has been completed in the initial cycle.
There is no obvious excessive growth of side reaction layers or ion blocking characteristic signals, indicating that the carefully designed particle arrangement and interface structure can still maintain efficient transmission channels under long-term high load and high rate conditions.
Further analysis indicates that under fast charging conditions, ion diffusion rate becomes a limiting factor, and the design of this study successfully reduces this limitation, allowing ions to quickly pass through the electrolyte particle interface, improving utilization and reducing polarization.
Summary
This study has established a set of design criteria for achieving high energy density, fast charging, and long lifespan all solid state batteries, and demonstrated their effectiveness through experiments. By optimizing the combination of NMC positive electrode material and sulfide solid electrolyte (LPSC), distributing particles reasonably, controlling electrode porosity and thickness, excellent performance was achieved at 30 ℃ with high capacity (~150 mAh/g) and long lifespan (81% capacity retention after 3000 cycles) even when charged at 15 mA/cm2 (approximately 8C rate). Meanwhile, by increasing the thickness and load of the positive electrode, rapid charging in the high SOC range (10% -80%) can still be completed within 10 minutes through moderate heating (80 ℃).