Structural and Electrochemical Impacts of Mg/Mn Dual Dopants on the LiNiO2 Cathode in Li-Metal Batteries

Effect of Cobalt Substitution on the Activation of the LiNiO2 Discharge Reaction

Cobalt (Co) has been introduced to most of the practical Ni-rich layered positive electrode materials owing to its ability to stabilize the layered structure and lessen cation-mixing. However, it has been unclear whether a highly ordered structure is essential or Co addition itself has some effects. In this study, we synthesized Co-substituted LiNiO2 (LNO) with and without the introduction of cation-mixing to investigate the detailed effects of Co on crystal/local structures and electrochemical properties. It was found that the charge-discharge reversibility of LNO was enhanced by Co substitution with an additional discharge capacity at around 3.5 V, showing that the reaction at the end of discharge was activated. This behavior was observed in Co-substituted LNO even if cation-mixing was largely introduced, implying the intrinsic effect of Co on reversibility. Solid-state NMR results showed that the local structure in LNO with cation-mixing significantly changed after charge-discharge, whereas that of Co-substituted LNO hardly changed even when cation-mixing was introduced, which seems to be responsible for better reversibility. Density functional theory calculation also supports the positive effect of Co on lithium transportation at the end of discharge.

Mg/Al Double-Pillared LiNiO2 as a Co-Free Ternary Cathode Material Ensuring Stable Cycling at 4.6 V

Cobalt-free (Co-free) and nickel-rich (Ni-rich) cathode materials have attracted significant attention and undergone extensive studies due to their affordability and superior energy density. However, the commercialization of these Co-free materials is hindered by challenges such as cation disorder, irreversible phase changes, and inadequate high-voltage performance. To overcome these challenges, a Co-free ternary cathode material of Mg/Al double-pillared LiNiO2 (NMA) synthesized via a wet-coating and lithiation-sintering technique is proposed. Fundamental studies reveal that Mg and Al have the potential to form a distinctive double-pillar structure within the layered cathode, enhancing its structural stability. To be specific, the strategic placement of Mg and Al in Li and Ni layers, respectively, effectively reduces Li+/Ni2+ disorder and prevents irreversible phase transitions. Additionally, the inclusion of Mg and Al refines the primary grains and compacts the secondary grains in the cathode material, reducing stress from cyclic usage and preventing material cracking, thereby mitigating electrolyte erosion. As a result, NMA demonstrates exceptional electrochemical performance under a high charge cutoff voltage of 4.6 V. It maintains 70% of initial specific capacity after 500 cycles at 1 C and exhibits excellent rate performance, with a capacity of 162 mAh g-1 at 5 C and 149 mAh g-1 at 10 C. As a whole, the produced NMA achieves a high structural stability in cases of excessive delithiation, providing a groundbreaking solution for the development of cost-effective and high-energy-density cathode materials for lithium-ion batteries.

High-energy all-solid-state lithium batteries enabled by Co-free LiNiO2 cathodes with robust outside-in structures

A critical current challenge in the development of all-solid-state lithium batteries (ASSLBs) is reducing the cost of fabrication without compromising the performance. Here we report a sulfide ASSLB based on a high-energy, Co-free LiNiO2 cathode with a robust outside-in structure. This promising cathode is enabled by the high-pressure O2 synthesis and subsequent atomic layer deposition of a unique ultrathin LixAlyZnzOδ protective layer comprising a LixAlyZnzOδ surface coating region and an Al and Zn near-surface doping region. This high-quality artificial interphase enhances the structural stability and interfacial dynamics of the cathode as it mitigates the contact loss and continuous side reactions at the cathode/solid electrolyte interface. As a result, our ASSLBs exhibit a high areal capacity (4.65 mAh cm-2), a high specific cathode capacity (203 mAh g-1), superior cycling stability (92% capacity retention after 200 cycles) and a good rate capability (93 mAh g-1 at 2C). This work also offers mechanistic insights into how to break through the limitation of using expensive cathodes (for example, Co-based) and coatings (for example, Nb-, Ta-, La- or Zr-based) while still achieving a high-energy ASSLB performance.

Ga-Doped Ultrahigh-Nickel Oxide Microspheres with Radially Aligned Primary Grains as a Cathode for Stable Cycling Li-Ion Batteries

Owing to the high energy density, ultrahigh-nickel (Ni > 0.9) layered oxides are used as promising cathode materials for next-generation Li-ion batteries. Unfortunately, the serious pulverization and rapid capacity fading during cycling limit the commercial viability of an ultrahigh-nickel oxide cathode. Herein, the introduction of Ga into LiNi0.96Co0.04O2 brings a radially aligned microstructural change of oxide microspheres during the lithiation of the Ni0.96Co0.04(OH)2 precursor. As expected, such radially aligned needle-like primary grains on microspheres have a positive influence to reduce the anisotropic volume change and suppress the formation of microcracks of Ga-induced Li(Ni0.96Co0.04)0.99Ga0.01O2 during cycling. Specifically, compared with irregular primary grains of LiNi0.96Co0.04O2, Ga-induced oxide presents a high initial discharge capacity of 227.9 mA h g-1 at 0.1C rate between 2.8 and 4.3 V. Especially, Ga-induced oxide delivers higher initial discharge capacities of 233.9 and 240.3 mA h g-1 with higher cutoff charge voltages of 4.4 and 4.5 V at 0.1C, respectively. Furthermore, a good capacity retention of 74.1% at 1 C rate is obtained after 300 cycles, which is almost 85% higher than that of the pristine sample, mainly due to the generation of microcracks of oxide microspheres during the long-term cycle. Therefore, the introduction of Ga into LiNi0.96Co0.04O2 is a feasible approach for improving the microstructure and cycling stability of the ultrahigh-Ni layered oxides.

Correlation of aluminum doping and lithiation temperature with electrochemical performance of LiNi1-xAlxO2 cathode material

This article presents a process for producing LiNi1-xAlxO2 (0 <  ×  < 0.05) cathode material with high capacity and enhanced cycle properties of 145 mAh/g after 600 cycles. The LiNi1-xAlxO2 (0 <  ×  < 0.05) cathode material is prepared by mixing coprecipitated Ni(OH)2 with LiOH and Al(OH)3, followed by lithiation at temperature range of 650–710 °C, after which any residual lithium from lithiation is washed from the particle surfaces. Electrochemical performance was studied within full-cell and half-cell application; in addition, different material characterization methods were carried out to explain structure changes when certain amount of aluminum is introduced in the LiNi1-xAlxO2 structure. Surface analyses were carried out to demonstrate how washing process changes the chemical environment of the LiNi1-xAlxO2 secondary particle surface. The results demonstrate how Al doping, lithiation temperature, and the washing process affect the performance of the LiNi1-xAlxO2 cathode material.

Compositionally complex doping for zero-strain zero-cobalt layered cathodes

The high volatility of the price of cobalt and the geopolitical limitations of cobalt mining have made the elimination of Co a pressing need for the automotive industry¹. Owing to their high energy density and low-cost advantages, high-Ni and low-Co or Co-free (zero-Co) layered cathodes have become the most promising cathodes for next-generation lithium-ion batteries^2,3. However, current high-Ni cathode materials, without exception, suffer severely from their intrinsic thermal and chemo-mechanical instabilities and insufficient cycle life. Here, by using a new compositionally complex (high-entropy) doping strategy, we successfully fabricate a high-Ni, zero-Co layered cathode that has extremely high thermal and cycling stability. Combining X-ray diffraction, transmission electron microscopy and nanotomography, we find that the cathode exhibits nearly zero volumetric change over a wide electrochemical window, resulting in greatly reduced lattice defects and local strain-induced cracks. In-situ heating experiments reveal that the thermal stability of the new cathode is significantly improved, reaching the level of the ultra-stable NMC-532. Owing to the considerably increased thermal stability and the zero volumetric change, it exhibits greatly improved capacity retention. This work, by resolving the long-standing safety and stability concerns for high-Ni, zero-Co cathode materials, offers a commercially viable cathode for safe, long-life lithium-ion batteries and a universal strategy for suppressing strain and phase transformation in intercalation electrodes.

Elucidating the Humidity-Induced Degradation of Ni-Rich Layered Cathodes for Li-Ion Batteries

Ni-rich layered oxides, in a general term of Li(Ni_xCo_yMn_1-x-y)O₂ (x > 0.5), are widely recognized as promising candidates for improving the specific energy and lowering the cost for next-generation Li-ion batteries. However, the high surface reactivity of these materials results in side reactions during improper storage and notable gas release when the cell is charged beyond 4.3 V vs Li⁺/Li⁰. Therefore, in this study, we embark on a comprehensive investigation on the moisture sensitivity of LiNi_0.85Co_0.1Mn_0.05O₂ by aging it in a controlled environment at a constant room-temperature relative humidity of 63% up to 1 year. We quantitatively analyze the gassing of the aged samples by online electrochemical mass spectrometry and further depict plausible reaction pathways, accounting for the origin of the gas release. Transmission electron microscopy reveals formation of an amorphous surface impurity layer of ca. 10 nm in thickness, as a result of continuous reactions with moisture and CO₂ from the air. Underneath it, there is another reconstructed layer of ca. 20 nm in thickness, showing rock salt/spinel-like features. Our results provide insight into the complex interfacial degradation phenomena and future directions for the development of high-performance Ni-rich layered oxides.

High Nickel and No Cobalt─The Pursuit of Next-Generation Layered Oxide Cathodes

The prosperity of the electric vehicle industry is driving the research and development of lithium-ion batteries. As one of the core components in the entire battery system, cathode materials are currently facing major challenges in pushing a higher capacity up to the materials' theoretical limits and transitioning away from unaffordable metals. The search for next-generation cathode materials has shifted to high-nickel and cobalt-free cathodes to meet these requirements. In this review, we distinctly point out the shortcomings of cobalt in stabilizing layered structures and systematically summarize the recent efforts to eliminate cobalt and achieve higher nickel content in layered cathode materials. Finally, a reasonable prospect is put forward for further development of layered cathode materials and other promising candidates, which is likely to spur a wave of efforts toward developing high-performance and low-cost Li-ion batteries.

Highly reversible Li-ion full batteries using a Mg-doped Li-rich Li1.2Ni0.28Mn0.468Mg0.052O2 cathode and carbon-decorated Mn3O4 anode with hierarchical microsphere structures

Microsphere structured Mg-doped Li-rich Li1.2Ni0.28Mn0.468Mg0.052O2 cathode and carbon-decorated Mn3O4 anode materials were prepared for application to lithium-ion full batteries. As-assembled lithium-ion full batteries exhibited enhanced electrochemical performances like high charge/discharge capacity, and long-term capacity retention.

Chemomechanically Stable Ultrahigh-Ni Single-Crystalline Cathodes with Improved Oxygen Retention and Delayed Phase Degradations

The pressing demand in electrical vehicle (EV) markets for high-energy-density lithium-ion batteries (LIBs) requires further increasing the Ni content in high-Ni and low-Co cathodes. However, the commercialization of high-Ni cathodes is hindered by their intrinsic chemomechanical instabilities and fast capacity fade. The emerging single-crystalline strategy offers a promising solution, yet the operation and degradation mechanism of single-crystalline cathodes remain elusive, especially in the extremely challenging ultrahigh-Ni (Ni > 90%) regime whereby the phase transformation, oxygen loss, and mechanical instability are exacerbated with increased Ni content. Herein, we decipher the atomic-scale stabilization mechanism controlling the enhanced cycling performance of an ultrahigh-Ni single-crystalline cathode. We find that the charge/discharge inhomogeneity, the intergranular cracking, and oxygen-loss-related phase degradations that are prominent in ultrahigh-Ni polycrystalline cathodes are considerably suppressed in their single-crystalline counterparts, leading to improved chemomechanical and cycling stabilities of the single-crystalline cathodes. Our work offers important guidance for designing next-generation single-crystalline cathodes for high-capacity, long-life LIBs.

Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries

Layered lithium nickel, manganese, and cobalt oxides (NMC) are among the most promising commercial positive electrodes in the past decades. Understanding the detailed surface and bulk redox processes of Ni-rich NMC can provide useful insights into material design options to boost reversible capacity and cycle life. Both hard X-ray absorption (XAS) of metal K-edges and soft XAS of metal L-edges collected from charged LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) showed that the charge capacity up to removing ∼0.7 Li/f.u. was accompanied with Ni oxidation in bulk and near the surface (up to 100 nm). Of significance to note is that nickel oxidation is primarily responsible for the charge capacity of NMC622 and 811 up to similar lithium removal (∼0.7 Li/f.u.) albeit charged to different potentials, beyond which was followed by Ni reduction near the surface (up to 100 nm) due to oxygen release and electrolyte parasitic reactions. This observation points toward several new strategies to enhance reversible redox capacities of Ni-rich and/or Co-free electrodes for high-energy Li-ion batteries.

Synthesis of LiNiO2 at Moderate Oxygen Pressure and Long-Term Cyclability in Lithium-Ion Full Cells

The widespread adoption of electric vehicles necessitates higher-energy-density and longer-life cathode materials for Li-ion batteries. LiNiO₂ offers a higher energy density at a lower cost than other high-Ni-content cathodes containing additional transition-metal ions. However, detrimental phase transformations and impedance growth, resulting from structural defects formed during synthesis, lead to poor cyclability and limit the practical viability of LiNiO₂. Herein, we demonstrate a considerably improved cycle life for LiNiO₂ by synthesizing it under a pressurized oxygen environment. The capacity retention in pouch-type full cells with a graphite anode after 1000 cycles is increased from 59 to 76% by applying a mere 1.7 atm of oxygen pressure during the synthesis of LiNiO₂. With iodometric titration and inductively coupled plasma optical emission spectroscopy analysis, we provide clear evidence that oxygen pressure during synthesis reduces the occurrence of lattice oxygen vacancies and increases the content of Ni³⁺ in LiNiO₂, improving its structural integrity and cyclability. Post-mortem analysis of the cycled cathodes provides insights into the sources of degradation occurring during long-term cycling. This work demonstrates a practically viable, synthetic approach combined with doping and coating to achieve improved performance with high-Ni layered oxide materials. Furthermore, this work represents the first report of extended cycling of LiNiO₂ in pouch full cells with graphite anode and will, therefore, serves as an important benchmark for future research on LiNiO₂.

Complementary Effects of Mg and Cu Incorporation in Stabilizing the Cobalt-Free LiNiO2 Cathode for Lithium-Ion Batteries

Since the discovery of LiNiO₂ several decades ago, a new era of electric vehicles demanding high-energy-density lithium-ion batteries (LIBs) has recently rebooted the interest in this cathode material to eliminate the dependence on expensive and scarcely available cobalt. However, LiNiO₂ has been plagued by cycle instability, thermal instability, and air instability. We present here an exploration of the mutual interaction of magnesium and copper in stabilizing the cobalt-free LiNiO₂ cathode. Although Mg doping is beneficial for the robustness of the bulk structure of LiNiO₂, surface characterization results of Mg-doped LiNiO₂ implies the need for further surface protection. To that end, we have incorporated Cu in addition to Mg in that Cu stabilizes the surface of Mg-doped LiNiO₂ by forming a protective stable surface layer without harming the bulk. Notable variations of the surface residual lithium composition (Li_Li2CO3/Li_LiOH) along with the incorporation of stabilizers are also discussed. The harmony between Mg and Cu with as little as 0.5 atom % Mg and 0.3 atom % Cu significantly enhances the specific energy and cycle life of LiNiO₂. This study demonstrates how the co-incorporation of optimal dopants can help stabilize both the bulk and surface and provides new insights toward developing cobalt-free layered oxide cathodes for high-energy-density LIBs.

High-Nickel NMA: A Cobalt-Free Alternative to NMC and NCA Cathodes for Lithium-Ion Batteries

High-nickel LiNi_{1- x - y} Mn_x Co_y O₂ (NMC) and LiNi_{1- x - y} Co_x Al_y O₂ (NCA) are the cathode materials of choice for next-generation high-energy lithium-ion batteries. Both NMC and NCA contain cobalt, an expensive and scarce metal generally believed to be essential for their electrochemical performance. Herein, a high-Ni LiNi_{1- x - y} Mn_x Al_y O₂ (NMA) cathode of desirable electrochemical properties is demonstrated benchmarked against NMC, NCA, and Al-Mg-codoped NMC (NMCAM) of identical Ni content (89 mol%) synthesized in-house. Despite a slightly lower specific capacity, high-Ni NMA operates at a higher voltage by ≈40 mV and shows no compromise in rate capability relative to NMC and NCA. In pouch cells paired with graphite, high-Ni NMA outperforms both NMC and NCA and only slightly trails NMCAM and a commercial cathode after 1000 deep cycles. Further, the superior thermal stability of NMA to NMC, NCA, and NMCAM is shown using differential scanning calorimetry. Considering the flexibility in compositional tuning and immediate synthesis scalability of high-Ni NMA very similar to NCA and NMC, this study opens a new space for cathode material development for next-generation high-energy, cobalt-free Li-ion batteries.