As a researcher in the field of energy storage, I have witnessed the growing demand for high-energy-density and safe battery systems. Conventional lithium-ion batteries with organic liquid electrolytes face significant challenges, including flammability, corrosion, and safety risks due to lithium dendrite growth. Solid-state batteries, which utilize solid electrolytes and lithium metal anodes, represent the next generation of rechargeable batteries with enhanced safety and energy density. The energy density of solid-state batteries can be 20–30% higher than that of liquid electrolyte-based systems when paired with the same cathode materials. However, the limited capacity of conventional cathode materials restricts further improvements. Li-rich Mn-based layered oxide (LRMO) cathodes, with their high specific capacity (≥250 mA·h·g–1) and energy density (≥1000 W·h·kg–1), are pivotal for achieving solid-state batteries with energy densities exceeding 600 W·h·kg–1. These cathodes leverage both transition metal cation and anion redox reactions for charge compensation, offering a cost-effective and environmentally friendly alternative due to their low cobalt and nickel content and high manganese abundance. In solid-state batteries, LRMO cathodes can mitigate transition metal dissolution and enhance structural stability, making them ideal for high-performance applications.
The structure of LRMO cathodes is composed of two phases: Li2MnO3 and LiMO2 (M = Mn, Ni, Co), forming a solid solution with a layered α-NaFeO2-type rock-salt structure. The Li2MnO3 component can be represented as Li[LixMn1−x]O2, where lithium atoms replace some manganese atoms in the transition metal layer, creating a honeycomb structure. This unique configuration results in unhybridized O 2p states, enabling oxygen redox activity. The electrochemical performance of LRMO cathodes involves two distinct regions during the first charge: a sloping region corresponding to lithium extraction from the LiMO2 structure with transition metal oxidation, and a plateau above 4.45 V where Li2MnO3 is activated, leading to oxygen oxidation and O2 release. The net removal of Li2O causes irreversible capacity loss, resulting in low initial Coulombic efficiency. However, the formed vacancies allow reversible lithium intercalation in subsequent cycles, contributing to the high capacity. The voltage decay during cycling is attributed to phase transitions from layered to spinel-like and rock-salt structures, driven by oxygen vacancies and manganese migration. In solid-state batteries, these issues are exacerbated by poor ionic/electronic conductivity and interfacial incompatibilities, necessitating strategic modifications.
The application of LRMO cathodes in solid-state batteries faces several challenges. The biphasic structure leads to low electronic conductivity and sluggish kinetics, limiting the activation of anion redox reactions. Chemical potential mismatches at the cathode-solid electrolyte interface cause spontaneous reactions, forming mixed conductive interphases that increase impedance. Oxygen release at high voltages oxidizes solid electrolytes, further degrading interface stability. To address these, researchers have developed bulk and interfacial modification strategies, including element doping, surface coating, and single-crystal engineering. For instance, doping with ruthenium enhances oxygen stability and lithium diffusion, while surface coatings like Li3PO4 or Li2B4O7 improve ionic transport and suppress interfacial degradation. Mechanical ball milling and single-crystal designs reduce grain boundaries and crack formation, enhancing mechanical stability and lithium ion pathways. These approaches have demonstrated improved performance in sulfide, halide, polymer, and oxide-based solid-state batteries, as summarized in Table 1.
| LRMO Cathode | Electrolyte Type | Initial Discharge Capacity / CE | Cycling Performance | Temperature (°C) |
|---|---|---|---|---|
| Li1.2Ni0.13Co0.13Mn0.54O2 | Sulfide | ~225 mA·h·g–1 / ~70% | 0.5 C, 1000 cycles, 83% retention | 27 |
| Li1.2Ni0.13Co0.13Mn0.54O2 | Sulfide | ~220 mA·h·g–1 / ~67.7% | 0.5 C, 1000 cycles, 87% retention | 27 |
| Li1.2Ni0.13Co0.13Mn0.54O2 | Sulfide | 204 mA·h·g–1 / 67.8% | 1 C, 2022 cycles, 72% retention | 55 |
| Li1.2Ni0.13Co0.13Mn0.54O2 | Halide | 248 mA·h·g–1 / 94% | 1 C, 300 cycles, 81.2% retention | 25 |
| Li1.2Ni0.13Co0.13Mn0.54O2 | Halide | 230 mA·h·g–1 / 83% | 0.5 C, 431 cycles, 60% retention | 25 |
| Li1.2Ni0.13Co0.13Mn0.54O2 | Halide | 231 mA·h·g–1 / 90% | 1 C, 2000 cycles, 80.4% retention | 25 |
| Li1.2Ni0.13Co0.13Mn0.54O2 | Halide | 244 mA·h·g–1 / 73% | 1 C, 750 cycles, 88.6% retention | 45 |
| Li1.2Ni0.13Co0.13Mn0.54O2 | Halide | 235.4 mA·h·g–1 / 74.83% | 1 C, 1200 cycles, 84.1% retention | 25 |
| Li1.2Ni0.2Mn0.6O2 | Polymer | ~245 mA·h·g–1 / 84% | 1 C, 200 cycles, 82.7% retention | 25 |
| Li1.2Ni0.16Co0.08Mn0.56O2 | Polymer | N.A. | 1 C, 400 cycles, 84.8% retention | 25 |
| Li1.2Ni0.2Mn0.6O2 | Polymer | N.A. | 0.2 C, 200 cycles, 91% retention | 30 |
| Li1.2Ni0.167Co0.067Mn0.567O2 | Oxide | 226 mA·h·g–1 / N.A. | 30 cycles, N.A. | 80 |
| Li1.2Ni0.2Mn0.6O2 | Oxide | 245 mA·h·g–1 / N.A. | 0.2 C, 200 cycles, 100% retention | 25 |
The electrochemical behavior of LRMO cathodes can be described using the following equations for the redox reactions. During charging, lithium extraction occurs as:
$$ \text{LiMO}_2 \rightarrow \text{Li}_{1-x}\text{MO}_2 + x\text{Li}^+ + x e^- $$
For the Li2MnO3 component, activation involves:
$$ \text{Li}_2\text{MnO}_3 \rightarrow 2\text{Li}^+ + \text{MnO}_2 + \frac{1}{2}\text{O}_2 + 2 e^- $$
The overall capacity contribution from anion redox can be modeled as:
$$ Q_{\text{total}} = Q_{\text{cation}} + Q_{\text{anion}} $$
where \( Q_{\text{cation}} \) is the capacity from transition metal oxidation and \( Q_{\text{anion}} \) is from oxygen redox. The voltage profile during cycling often shows decay due to phase transitions, which can be expressed as a function of cycle number \( n \):
$$ V(n) = V_0 – k \log(n) $$
where \( V_0 \) is the initial voltage and \( k \) is a decay constant.
In solid-state batteries, the interface between LRMO cathodes and solid electrolytes is critical. The ionic conductivity \( \sigma_i \) at the interface can be affected by space charge layers, given by:
$$ \sigma_i = \sigma_0 \exp\left(-\frac{E_a}{kT}\right) $$
where \( \sigma_0 \) is the pre-exponential factor, \( E_a \) is the activation energy, \( k \) is Boltzmann’s constant, and \( T \) is temperature. To stabilize the interface, coatings such as Li3PO4 reduce the activation energy, enhancing lithium ion transport.
Polycrystalline LRMO cathodes, consisting of aggregated nano-sized primary particles, suffer from intergranular cracks and poor ionic pathways in solid-state batteries. Modifications like carbon nanotube networks and atomic layer deposition of Li3PO4 have improved electronic conductivity and interface stability. For example, in halide-based solid-state batteries, these strategies achieved initial discharge capacities of 230.7 mA·h·g–1 and stable cycling over 100 cycles. Dual-functional approaches, such as ruthenium doping and surface sulfidation, suppress oxygen release and enhance kinetics, enabling long-term stability at high temperatures (55°C) with over 70% capacity retention after 2022 cycles.
Mechanochemical methods, including ball milling, break down secondary particles into primary ones and incorporate fast ion conductors like Li2B4O7, creating 3D lithium ion networks. This improves the reversibility of anion redox reactions, as seen in halide-based solid-state batteries with initial Coulombic efficiencies of 94% and capacities around 248 mA·h·g–1. The introduction of boron strengthens B–O bonds, stabilizing the oxygen lattice and enabling 2000 cycles with 80.6% capacity retention.
Single-crystal LRMO cathodes, with minimal grain boundaries, offer superior mechanical strength and reduced cracking. Sub-micron single crystals with integrated Li2WO4 phases enhance bulk ionic conductivity and interface stability. For instance, in halide-based systems, these cathodes delivered areal capacities ≥3.15 mA·h·cm–2 and 84.1% retention after 1200 cycles at 1 C. Multifunctional surface modifications, such as lithium gradient layers and Li2MoO4 coatings, further promote interface lithium ion transport without compromising oxygen redox activity, achieving 244 mA·h·g–1 at 0.05 C and stable performance at 45°C.
Reducing particle size to sub-micron scales in single-crystal LRMO cathodes optimizes interface contact and shortens diffusion paths. In sulfide-based solid-state batteries, this resulted in reversible capacities of 316 mA·h·g–1 at 0.05 C and 210 mA·h·g–1 at 1 C, with 86% retention after 300 cycles. High-loading electrodes (20 mg·cm–2) maintained 296 mA·h·g–1 and 90% retention over 50 cycles, highlighting the importance of morphology control.
Despite progress, challenges remain in scaling up LRMO cathodes for solid-state batteries. The complex synthesis of single crystals requires optimization for high yield and quality. Zero-strain materials, which minimize volume changes during cycling, are essential to mitigate mechanical stress and cracking. Machine learning can accelerate the design of such materials and interface coatings by predicting stable structures and ionic conductivities. For example, algorithms can optimize cation ordering or coating thickness to enhance compatibility.
Developing high-voltage-tolerant solid electrolytes is crucial for matching LRMO cathodes. Advances in dry electrode technology and thin-film processing can enable large-format Ah-level cells. The integration of AI-driven models for material screening and interface engineering will be key to overcoming current limitations. For instance, predictive models can identify novel solid electrolytes with high oxidative stability at voltages above 4.5 V.
In conclusion, LRMO cathodes hold immense potential for high-energy-density solid-state batteries. Through bulk and interfacial engineering, single-crystal designs, and advanced manufacturing, these cathodes can overcome issues like voltage decay and oxygen release. Future research should focus on zero-strain materials, AI-guided optimization, and scalable production techniques to realize the full promise of solid-state batteries for applications in electric vehicles and grid storage. The synergy between materials science and engineering will drive the commercialization of next-generation energy storage systems.