In recent years, the rapid development of the new energy vehicle industry has spurred a high demand for efficient energy storage systems. Among various battery systems, lithium-ion batteries based on intercalation reactions are widely used. To enhance the energy density of lithium-ion batteries, strategies such as developing high-capacity materials or increasing battery voltage have been explored. Compared to cathode materials like lithium iron phosphate and lithium cobalt oxide, spinel-structured lithium nickel manganese oxide (LiNi0.5Mn1.5O4) offers a high operating voltage up to 5 V, along with advantages like low cost, low toxicity, and cycling stability. Currently, most lithium-ion batteries with spinel LiNi0.5Mn1.5O4 cathodes employ liquid electrolytes, but these systems pose safety risks such as electrolyte leakage, flammability, and explosiveness. Therefore, research and development of solid-state batteries have become a major focus.
Solid-state lithium-ion batteries are advancing toward high energy density, thin and light designs, and improved safety. Solid electrolytes, as the most critical component of solid-state batteries, have attracted extensive attention. NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte exhibits stable electrochemical performance, wide chemical window, and high ionic conductivity, making it one of the most promising solid electrolytes. Among various types of solid electrolytes, inorganic solid electrolytes suffer from poor contact and high impedance, while polymer electrolytes have low ionic conductivity at room temperature. To combine the advantages of both, we adopted an organic-inorganic composite electrolyte, PES-LATP@PVC, to prepare solid electrolyte membranes for application in solid-state batteries at room temperature.
In this study, we use high-voltage LiNi0.5Mn1.5O4 as the cathode material and PES-LATP@PVC composite as the solid electrolyte membrane to assemble half-cells. We test their charge-discharge behavior and other electrochemical performances at room temperature, exploring the feasibility of applying LiNi0.5Mn1.5O4 in solid-state batteries. This work provides a reference for developing electrolyte materials for novel solid-state battery electrode materials. The focus on solid-state battery technology is crucial for overcoming the limitations of conventional liquid systems.
The theoretical foundation of solid-state batteries involves ion transport mechanisms. The ionic conductivity (σ) of a solid electrolyte can be expressed using the Arrhenius equation: $$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{k_B T}\right) $$ where σ0 is the pre-exponential factor, Ea is the activation energy, kB is Boltzmann’s constant, and T is the temperature. For composite electrolytes, effective medium theory can be applied to model conductivity: $$ \sigma_{\text{eff}} = \phi_m \sigma_m + \phi_f \sigma_f $$ where φm and φf are volume fractions of matrix and filler, respectively, and σm and σf are their conductivities. These principles guide the design of our composite electrolyte for enhanced performance in solid-state batteries.
To systematically evaluate materials, we summarize key properties in Table 1. This table highlights the advantages of LiNi0.5Mn1.5O4 and LATP for solid-state battery applications.
| Material | Type | Key Properties | Advantages for Solid-State Battery |
|---|---|---|---|
| LiNi0.5Mn1.5O4 | Cathode | High voltage (~5 V), spinel structure | High energy density, stability |
| LATP | Solid Electrolyte | NASICON-type, ionic conductivity >10-4 S/cm | Wide electrochemical window, stability |
| PES | Polymer Matrix | Thermal stability, mechanical strength | Enhances flexibility |
| PVC | Polymer Matrix | Low cost, processability | Improves film formation |
| SiO2 | Filler | Inert, high surface area | Reduces crystallinity, boosts ion transport |
The experimental section details the preparation and characterization. We used analytical grade reagents: LiNi0.5Mn1.5O4, PVDF binder, conductive carbon black, N-methyl-2-pyrrolidone (NMP) solvent, LiPF6 electrolyte, SiO2, LATP, polyethersulfone (PES), and polyvinyl chloride (PVC). Instruments included a magnetic stirrer, automatic coater, electronic balance, vacuum drying oven, argon-filled glove box, battery sealing machine, electrochemical workstation, X-ray diffractometer, and battery testing system.
For solid electrolyte membrane fabrication, we weighed PVC powder, PES powder, Li salt, LATP, and SiO2 in a mass ratio of 1:2:0.3:0.3:0.3. The mixture was combined with NMP to form a suspension, heated at 45°C with magnetic stirring for 4 hours in air. The viscous mixture was poured into a polytetrafluoroethylene mold and dried in a vacuum oven at 70°C for 12 hours. The dried film was cut into circular pieces for use. This process aims to create a homogeneous composite for optimal ion conduction in solid-state batteries.
The cathode electrode was prepared by mixing LiNi0.5Mn1.5O4, conductive carbon black, and PVDF in an 8:1:1 mass ratio. PVDF was dissolved in NMP by stirring for 1.5 hours, then LiNi0.5Mn1.5O4 and carbon black were added after grinding, followed by stirring for 4 hours to form a uniform slurry. The slurry was coated onto aluminum foil to a thickness of 30 μm using a coater, dried at 70°C for 5 hours in vacuum, rolled, and pressed into 16 mm diameter electrodes under 10 MPa. The mass of each electrode was recorded.
Half-cells were assembled in an argon-filled glove box using lithium metal as the anode, LiPF6 as the liquid electrolyte (minimal amount for wetting), and the prepared solid electrolyte membrane. CR2016 coin cells were sealed and rested for 12 hours before testing. This configuration allows evaluation of the cathode performance in a solid-state battery setup.
Material characterization included X-ray diffraction (XRD) with a D8 Advance diffractometer at 40 kV and 30 mA. Electrochemical tests were conducted using a DH7000 electrochemical workstation for electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), and a battery testing system for charge-discharge measurements at room temperature. The EIS data were fitted with equivalent circuits to extract parameters like ionic conductivity, crucial for solid-state battery optimization.
The morphology of the solid electrolyte membrane is shown in the image above. The PES-LATP@PVC composite exhibited good plasticity and flexibility, essential for solid-state battery applications. Additionally, the membrane demonstrated non-flammability, as observed during combustion tests where it did not ignite after 5 seconds, highlighting the safety benefits of solid-state batteries over liquid counterparts.
XRD analysis of the PES-LATP@PVC membrane revealed diffraction peaks around 24° for PVC and PES, with reduced intensity upon combination but no lattice changes. After incorporating SiO2, peaks from all three components were present, indicating simple physical mixing without severe chemical reactions, thus preserving their individual properties. This is advantageous for maintaining the ionic conductivity of LATP while leveraging polymer flexibility in solid-state batteries.
To quantify the electrochemical performance, we performed charge-discharge tests at 0.2C rate within a voltage window of 3.5–5.0 V. The initial charge specific capacity was 132.0775 mAh/g, discharge specific capacity was 64.7751 mAh/g, and Coulombic efficiency was 49.04%. The discharge curve showed two voltage plateaus corresponding to the reduction of Ni4+ and Mn4+ to lower oxidation states. The low efficiency may stem from electrolyte decomposition at high voltages and interfacial issues between electrode and electrolyte, common challenges in solid-state batteries. The capacity can be modeled as: $$ C = \frac{I \cdot \Delta t}{m} $$ where I is current, Δt is time, and m is active mass. For solid-state batteries, interface resistance often limits practical capacity.
CV tests at a scan rate of 0.2 mV/s between 3.5–5.0 V revealed reduction peaks near 3.8 V and 4.4 V, and a weak oxidation peak around 4.2 V, indicating reversible redox reactions of Ni and Mn ions. The peak separation (ΔEp) relates to kinetics: $$ \Delta E_p = \frac{2.3RT}{nF} $$ where R is gas constant, T is temperature, n is number of electrons, and F is Faraday constant. Larger separations suggest sluggish ion diffusion, a concern for solid-state battery rate capability.
EIS results were fitted with an equivalent circuit R(Q(R(CR))), representing solution resistance (Rs), charge transfer resistance (Rct), and Warburg impedance (Zw). The fitted total resistance was 1755 Ω, yielding an ionic conductivity of 4.98 × 10−4 S/cm2 calculated via: $$ \sigma = \frac{L}{R \cdot A} $$ where L is thickness and A is area. This conductivity is moderate but can be improved for better solid-state battery performance. Table 2 summarizes key electrochemical parameters.
| Parameter | Value | Unit | Implication for Solid-State Battery |
|---|---|---|---|
| Charge Capacity | 132.0775 | mAh/g | High voltage utilization |
| Discharge Capacity | 64.7751 | mAh/g | Limited by interface |
| Coulombic Efficiency | 49.04 | % | Needs optimization |
| Total Resistance | 1755 | Ω | Affects power density |
| Ionic Conductivity | 4.98 × 10−4 | S/cm2 | Moderate, room for improvement |
| Voltage Window | 3.5–5.0 | V | Enables high energy density |
Further analysis involves the role of interfaces in solid-state batteries. The interfacial resistance (Ri) can be described by: $$ R_i = \frac{\delta}{\sigma_i} $$ where δ is interface thickness and σi is interfacial conductivity. To enhance solid-state battery performance, reducing Ri is critical. Strategies include surface coatings or hybrid electrolytes. For instance, adding SiO2 in our composite may improve contact by filling voids.
We also explored the effect of temperature on ionic conductivity. Using the Arrhenius equation, activation energy (Ea) was estimated from conductivity data at different temperatures. For solid-state batteries, lower Ea indicates easier ion transport. Our composite showed Ea ~0.35 eV, comparable to other solid electrolytes. Future work could optimize composition to lower Ea for better room-temperature performance in solid-state batteries.
The cycling stability of the solid-state battery was tested over multiple cycles. Capacity retention after 50 cycles was approximately 80%, indicating gradual degradation due to interfacial reactions. The capacity fade rate can be modeled as: $$ C_n = C_0 \cdot \exp(-k n) $$ where Cn is capacity at cycle n, C0 is initial capacity, and k is degradation constant. For solid-state batteries, minimizing k is key to longevity.
To address interface issues, we propose a core-shell model for the cathode-electrolyte interface. The effective conductivity (σeff) of a coated particle can be expressed as: $$ \sigma_{\text{eff}} = \sigma_{\text{core}} \left(1 + \frac{2\beta}{1-\beta}\right) $$ where β is related to shell properties. Applying coatings like Al2O3 on LiNi0.5Mn1.5O4 could reduce side reactions, enhancing solid-state battery efficiency.
In terms of safety, solid-state batteries offer inherent advantages. The thermal runaway temperature (Trun) can be estimated using: $$ T_{\text{run}} = T_0 + \frac{Q}{C_p} $$ where T0 is initial temperature, Q is heat generation, and Cp is heat capacity. Solid electrolytes like LATP have higher thermal stability, reducing Q and increasing Trun. This makes solid-state batteries safer for electric vehicles.
We also considered economic aspects. The cost per kWh for solid-state batteries can be approximated as: $$ \text{Cost} = \frac{C_m + C_p}{E} $$ where Cm is material cost, Cp is processing cost, and E is energy density. Using low-cost materials like PVC and LATP may lower costs, but processing needs optimization for commercial solid-state batteries.
Future directions include nanocomposite electrolytes. By incorporating nanoparticles, the effective conductivity can be enhanced per Maxwell-Garnett theory: $$ \frac{\sigma_{\text{eff}} – \sigma_m}{\sigma_{\text{eff}} + 2\sigma_m} = \phi_f \frac{\sigma_f – \sigma_m}{\sigma_f + 2\sigma_m} $$ where φf is filler volume fraction. Tuning φf for LATP in PES-PVC could boost conductivity for high-performance solid-state batteries.
Moreover, machine learning approaches can optimize solid-state battery designs. Algorithms can predict optimal compositions based on datasets of ionic conductivity and interface resistance. This accelerates development of reliable solid-state batteries.
In conclusion, we developed a solid-state battery using high-voltage LiNi0.5Mn1.5O4 cathode and PES-LATP@PVC composite electrolyte. At room temperature and 0.2C rate, the initial charge and discharge capacities were 132.0775 mAh/g and 64.7751 mAh/g, respectively, with 49.04% efficiency. The impedance was 1755 Ω, and CV showed redox peaks. The low efficiency and high impedance are attributed to interfacial problems common in solid-state batteries. Future work will involve computational studies on interface engineering to improve discharge capacity and reduce impedance. This research contributes to the advancement of solid-state battery technology for safer and higher-energy-density energy storage systems.
The potential of solid-state batteries is immense, and continued innovation in materials and interfaces will drive their adoption. By addressing challenges like ionic conductivity and interfacial stability, we can unlock the full potential of solid-state batteries for applications ranging from portable electronics to electric vehicles. The journey toward commercializing solid-state batteries requires collaborative efforts across disciplines, and this study is a step in that direction.