Lithium-Rich Manganese-Based Materials
Advanced cathode materials for next-generation energy storage systems, addressing performance challenges while considering safety aspects related to lithium ion battery fires.
1. Composition and Structure
Lithium-rich manganese-based materials are composite cathode materials based on Li₂MnO₃, formulated as Li₂MnO₃:LiMO₂ (where M is typically Ni, Co, Mn, or binary or ternary layered materials of Ni, Co, and Mn). Compared to LiMn₂O₄ or pure layered LiMnO₂ cathode materials, these materials have a higher Li/M molar ratio, and are generally referred to as layered lithium-rich manganese-based compounds. Researchers have studied various systems including Li₂MnO₃·LiCoO₂, Li₂MnO₃·LiNi₁₋ₓCoₓO₂, xLi₂MnO₃·(1-x)LiNi₀.₅Mn₀.₅O₂, and xLi₂MnO₃·(1-x)LiNi₁/3Co₁/3Mn₁/3O₂, with safety considerations including prevention of lithium ion battery fires being an important research aspect.
The structure of Li₂MnO₃·LiMO₂ contains two components: LiMO₂ and Li₂MnO₃. Among these, the LiMO₂ layered cathode material belongs to the R3m space group. The crystal structure of Li₂MnO₃ is similar to LiMO₂, with the difference being that the transition metal layer contains Li. Li and Mn occupy the M layer in an atomic ratio of 1:2, where each Li is surrounded by six Mn atoms, forming a structure as shown in Figure 2-5. This corresponds to a characteristic peak near the 20° diffraction angle in the XRD pattern. Therefore, Li₂MnO₃ can also be written in the form of Li[Li₁/3Mn₂/3]O₂. However, due to reduced structural symmetry, the superstructure formed by Li⁺ and Mn⁴⁺ in the transition metal layer reduces the lattice symmetry of Li₂MnO₃ from the R3m space group to the monoclinic C2/m space group, with a=0.4937nm, b=0.8532nm, c=0.5030nm, and β=109.46°. These structural characteristics play a crucial role in battery safety and can influence the risk of lithium ion battery fires under certain conditions.
Figure 2-5: Structural diagrams of Li₂MnO₃ and LiMO₂
(a) Li₂MnO₃ Structure
Octahedral MO₆ arrangement with Li ions in specific lattice positions
(b) LiMO₂ Structure
Layered structure with alternating Li and transition metal layers
Because Li₂MnO₃ and LiMO₂ form a host lattice in a cubic close-packed arrangement with the same oxygen ion arrangement and a layer spacing of 0.47nm, the two separate phases cannot be distinguished by high-resolution transmission electron microscopy. This structural integration affects not only electrochemical performance but also thermal stability, a key factor in understanding and preventing lithium ion battery fires.
There is currently no consensus on whether lithium-rich manganese-based materials are solid solutions of Li₂MnO₃ and LiMO₂. XRD analysis has found a linear relationship between lattice parameters and composition, showing significant solid solution characteristics. However, direct observation by HRTEM has revealed that the material is formed by alternating nanodomains of Li₂MnO₃ and LiMO₂. Comprehensive analysis has shown that the distribution of lithium and transition metal elements within the transition metal layer of lithium-rich manganese-based materials is statistically uniform, forming a uniform solid solution on the micrometer scale. However, on the nanoscale, it appears to be a two-phase mixture of Li₂MnO₃ and LiMO₂, as shown in Figure 2-6. These nanoscale features can influence thermal runaway behavior, which is central to understanding lithium ion battery fires.
Figure 2-6: Schematic diagram of two-phase mixture of Li₂MnO₃ and LiMO₂
Li₂MnO₃ domains
Characterized by specific atomic arrangement and lattice parameters
LiMO₂ domains
Exhibiting layered structure with transition metal ions
The unique nanostructure of these materials contributes to their high capacity but also introduces challenges in terms of thermal management. Understanding how these phases interact under different temperature conditions is essential for developing safer battery systems with reduced risk of lithium ion battery fires. Research continues to explore how modifying the composition and processing conditions can optimize both performance and safety profiles.
Another important aspect of the composition is the role of dopants and substituents. Small amounts of certain elements can be introduced to stabilize the structure, improve conductivity, and enhance thermal stability. These modifications not only improve electrochemical performance but also play a role in mitigating risks associated with lithium ion battery fires by reducing the likelihood of thermal runaway reactions.
2. Electrochemical Performance
Li[Li₁/3Ni₁/3Mn₁/3]O₂ exhibits a unique charging plateau when first charged to 4.5V. During the second charge, the 4.5V plateau disappears, indicating that the 4.5V charging plateau is irreversible. The charge-discharge curves are shown in Figure 2-7. This irreversible behavior has implications for battery safety systems designed to prevent lithium ion battery fires, as it affects how the battery responds to repeated charge cycles.
Figure 2-7: Charge-discharge curves of typical lithium-rich materials
Comparison of first and subsequent cycles showing the characteristic 4.5V plateau in the initial charge that disappears in subsequent cycles, a factor that must be considered in thermal management systems to prevent lithium ion battery fires.
During the charging process of lithium-rich manganese-based materials, when the initial charging voltage is less than 4.5V, it corresponds to the charging process of the layered material LiMO₂. The plateau when charging above 4.5V indicates a new charge-discharge mechanism, corresponding to the charging process of Li₂MnO₃. While the charging mechanism of LiMO₂ has been discussed earlier, we will focus on the charging mechanism of Li₂MnO₃ here. The charging mechanism of Li₂MnO₃ mainly includes: oxygen evolution, proton exchange, and a mixed mechanism involving both processes. Understanding these mechanisms is crucial for developing battery management systems that can prevent conditions leading to lithium ion battery fires.
The oxygen evolution mechanism suggests that after charging above 4.5V, Li⁺ ions are extracted from the Li₂MnO₃ lattice, while O²⁻ ions leave the host lattice and are oxidized, equivalent to extracting Li₂O and leaving oxygen vacancies. This process includes both electrochemical and chemical steps: the electrochemical process involves Li₂MnO₃ losing Li⁺ ions while losing electrons, generating the intermediate state Mn⁴⁺O₂⁻, as shown in reaction formula (2-3); the chemical process involves unstable Mn⁴⁺O₂⁻ subsequently releasing oxygen, as shown in reaction formula (2-4). This oxygen release is a critical factor in battery safety, as released oxygen can accelerate reactions that lead to lithium ion battery fires.
Reaction Mechanisms
(2-3) Electrochemical process
Li₂MnO₃ → 2Li⁺ + Mn⁴⁺O₂⁻ + 2e⁻
(2-4) Chemical process
2Mn⁴⁺O₂⁻ → 2MnO₂ + O₂
(2-5) Proton exchange (without oxygen evolution)
Li₂MnO₃ + xH⁺ → Li₂₋ₓHₓMnO₃ + xLi⁺
(2-6) Proton exchange (with oxygen evolution)
Li₂MnO₃ + 2yH⁺ → Li₂₋₂yMnO₃₋y + 2yLi⁺ + yH₂O
(2-7) Discharge reaction
xMnO₂·(1-x)MO + Li → xLiMnO₂·(1-x)LiMO₂
The proton exchange mechanism suggests that Li₂MnO₃ undergoes a displacement reaction with H⁺ ions produced by electrolyte decomposition. One viewpoint suggests that oxygen ions in Li₂MnO₃ do not leave the lattice, as shown in reaction formula (2-5); another viewpoint suggests that Li₂MnO₃ undergoes displacement with H⁺ ions accompanied by oxygen ions leaving the lattice, as shown in reaction formula (2-6). These reactions can affect electrolyte stability and contribute to processes that may lead to lithium ion battery fires if not properly managed.
In fact, researchers tend to support a mixed mechanism combining both processes. The mixed mechanism suggests that oxygen evolution is dominant at low temperatures and during the early stages of charging, while proton exchange becomes dominant at high temperatures and during the middle and later stages of charging. This temperature dependence is particularly important for understanding thermal runaway scenarios that can result in lithium ion battery fires.
During the discharge process of lithium-rich manganese-based materials, during the first discharge and subsequent charge-discharge cycles, Li intercalates and deintercalates between the layered structures, undergoing reactions along path 3-4 in the Li₂MnO₃-LiMO₂-MO₂ phase diagram [Figure 2-8(a)] as described by reaction (2-7). This reaction pathway influences both capacity retention and thermal stability, with implications for preventing lithium ion battery fires.
Figure 2-8: Li₂MnO₃-LiMO₂-MO₂ phase diagram (a) and charge-discharge schematic for xLi₂MnO₃·(1-x)LiMO₂ materials (b)
(a) Phase Diagram
Phase relationships showing reaction pathways during charge-discharge cycles
(b) Charge-Discharge Schematic
Illustration of structural changes during electrochemical cycling
Therefore, in xLi₂MnO₃·(1-x)LiMO₂ composite materials, the initial charge-discharge cycle has a different curve shape compared to subsequent cycles, as shown in Figure 2-8(b). From the above analysis, it can be seen that Li₂MnO₃ needs to be activated above 4.5V to participate in lithium ion intercalation-deintercalation reactions. Therefore, the charge cut-off voltage for this material must be greater than 4.5V, with 4.8V commonly used in the literature. Since the tetravalent manganese in discharged Li₂MnO₃ participates in reactions below 3V, the discharge cut-off voltage must be below 3V, with 2V commonly used in the literature. These voltage parameters are critical for battery safety design, as improper voltage limits can increase the risk of lithium ion battery fires.
Lithium-rich cathode materials can achieve higher specific capacities over a wider voltage range, with practical specific capacities up to 220 mAh/g. During the first charge, two Li ions are extracted with Li₂O, and while one Li⁺ ion returns to the cathode during discharge, the other Li⁺ ion that does not return can be used to compensate for the irreversible capacity of the anode. This makes it possible to utilize Si-based and Sn-based anode materials with large irreversible capacities, but also introduces challenges in terms of battery management to prevent lithium ion battery fires.
The high capacity of these materials is accompanied by challenges such as voltage fade, capacity degradation, and thermal stability issues. Researchers are actively working on surface modifications, doping strategies, and nanostructuring to address these issues while maintaining high performance. These improvements not only enhance battery life but also contribute to reducing the risk of lithium ion battery fires by improving thermal stability and reducing side reactions.
Thermal stability testing is a critical aspect of lithium-rich manganese-based material development. Differential scanning calorimetry (DSC) studies show that these materials exhibit exothermic reactions at specific temperatures, which must be understood and mitigated in battery design. By optimizing the material composition and surface coatings, researchers aim to shift these exothermic reactions to higher temperatures, providing a larger safety margin and reducing the risk of lithium ion battery fires.
Another important consideration is the interaction between lithium-rich manganese-based cathodes and electrolytes. The high voltage operation can accelerate electrolyte decomposition, generating heat and gas that contribute to battery swelling and potential thermal runaway. Developing stable electrolytes and protective coatings for these high-voltage cathodes is essential for commercialization and for minimizing the risk of lithium ion battery fires in practical applications.
In summary, lithium-rich manganese-based materials offer significant promise for next-generation lithium-ion batteries with higher energy densities. However, their successful commercialization requires addressing not only performance challenges like voltage fade and rate capability but also critical safety considerations related to thermal stability and prevention of lithium ion battery fires. Continued research into their structure, electrochemical mechanisms, and thermal behavior will be essential for unlocking their full potential in energy storage applications.