Hyung-Joo Noh

Nanostructured high-energy cathode materials for advanced lithium batteries

Nickel-rich layered lithium transition-metal oxides, LiNi(1-x)M(x)O(2) (M = transition metal), have been under intense investigation as high-energy cathode materials for rechargeable lithium batteries because of their high specific capacity and relatively low cost. However, the commercial deployment of nickel-rich oxides has been severely hindered by their intrinsic poor thermal stability at the fully charged state and insufficient cycle life, especially at elevated temperatures. Here, we report a nickel-rich lithium transition-metal oxide with a very high capacity (215 mA h g(-1)), where the nickel concentration decreases linearly whereas the manganese concentration increases linearly from the centre to the outer layer of each particle. Using this nano-functional full-gradient approach, we are able to harness the high energy density of the nickel-rich core and the high thermal stability and long life of the manganese-rich outer layers. Moreover, the micrometre-size secondary particles of this cathode material are composed of aligned needle-like nanosize primary particles, resulting in a high rate capability. The experimental results suggest that this nano-functional full-gradient cathode material is promising for applications that require high energy, long calendar life and excellent abuse tolerance such as electric vehicles.

An effective method to reduce residual lithium compounds on Ni-rich Li[Ni0.6Co0.2Mn0.2]O2 active material using a phosphoric acid derived Li3PO4 nanolayer

The Ni-rich Li[Ni0.6Co0.2Mn0.2]O2 surface has been modified with H3PO4. After coating at 80 °C, the products were heated further at a moderate temperature of 500 °C in air, when the added H3PO4 transformed to Li3PO4 after reacting with residual LiOH and Li2CO3 on the surface. A thin and uniform smooth nanolayer (< 10 nm) was observed on the surface of Li[Ni0.6Co0.2Mn0.2]O2 as confirmed by transmission electron microscopy (TEM). Time-of-flight secondary ion mass spectroscopic (ToF-SIMS) data exhibit the presence of LiP+, LiPO+, and Li2PO2+ fragments, indicating the formation of the Li3PO4 coating layer on the surface of the Li[Ni0.6Co0.2Mn0.2]O2. As a result, the amounts of residual lithium compounds, such as LiOH and Li2CO3, are significantly reduced. As a consequence, the Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2 exhibits noticeable improvement in capacity retention and rate capability due to the reduction of residual LiOH and Li2CO3. Further investigation of the extensively cycled electrodes by X-ray diffraction (XRD), TEM, and ToF-SIMS demonstrated that the Li3PO4 coating layers have multi-functions: Absorption of water in the electrolyte that lowers the HF level, HF scavenging, and protection of the active materials from deleterious side reactions with the electrolyte during extensive cycling, enabling high capacity retention over 1,000 cycles.

Progress in High-Capacity Core–Shell Cathode Materials for Rechargeable Lithium Batteries

High-energy-density rechargeable batteries are needed to fulfill various demands such as self-monitoring analysis and reporting technology (SMART) devices, energy storage systems, and (hybrid) electric vehicles. As a result, high-energy electrode materials enabling a long cycle life and reliable safety need to be developed. To ensure these requirements, new material chemistries can be derived from combinations of at least two compounds in a secondary particle with varying chemical composition and primary particle morphologies having a core-shell structure and spherical cathode-active materials, specifically a nanoparticle core and shell, nanoparticle core and nanorod shell, and nanorod core and shell. To this end, several layer core-shell cathode materials were developed to ensure high capacity, reliability, and safety.

High-Voltage Performance of Li [ Ni0.55Co0.15Mn0.30 ] O2 Positive Electrode Material for Rechargeable Li-Ion Batteries

The electrochemical properties and thermal stabilities of a new positive electrode material for Li-ion batteries, Li[Ni 0.55 Co 0.15 Mn 0.30 ]O 2 , were investigated over a wide potential window. This electrode material was synthesized via a coprecipitation method. X-ray diffraction studies indicated that the synthesized material crystallized into an α-NaFeO 2 layered structure (R3m). The Li[Ni 0.55 Co 0.15 Mn 0.30 ]O 2 positive electrode has a discharge capacity of 202 mAh g -1 in the voltage range of 2.7-4.5 V. This high capacity was retained throughout cycling. The thermal stability of Li[Ni 0.55 Co 0.15 Mn 0.30 ]O 2 was measured by differential thermal calorimetry and found to be comparable to that of Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 . This positive electrode material was also characterized in a full cell configuration (graphite negative electrode) by the hybrid pulse power characterization tests following the FreedomCAR battery test manual for plug-in hybrid electric vehicles (PHEVs). The pulse power capability and available energy met the goals for PHEVs.

Comparison of Nanorod‐Structured Li[Ni0.54Co0.16Mn0.30]O2 with Conventional Cathode Materials for Li‐Ion Batteries

We successfully synthesized a safe, high-capacity cathode material specifically engineered for EV applications with a full concentration gradient (FCG) of Ni and Co ions at a fixed Mn content throughout the particles. The electrochemical and thermal properties of the FCG Li[Ni(0.54)Co(0.16)Mn(0.30)]O2 were evaluated and compared to those of conventional Li[Ni(0.5) Co(0.2) Mn(0.3)]O2 and Li[Ni(1/3)Co(1/3)Mn(1/3)]O2 materials. It was found that the FCG Li[Ni(0.54)Co(0.16)Mn(0.30)]O2 demonstrated a higher discharge capacity and a superior lithium intercalation stability compared to Li[Ni(0.5) Co(0.2)Mn(0.3)]O2 and Li[Ni(1/3)Co(1/3)Mn(1/3)]O2 over all of the tested voltage ranges. The results of electrochemical impedance spectroscopy and transition-metal dissolution demonstrate that the microstructure of primary particle with rod-shaped morphology plays an important role in reducing metal dissolution, which thereby decreases the charge transfer resistance as a result of stabilization of the host structure.

Nanorod and Nanoparticle Shells in Concentration Gradient Core–Shell Lithium Oxides for Rechargeable Lithium Batteries

The structure, electrochemistry, and thermal stability of concentration gradient core-shell (CGCS) particles with different shell morphologies were evaluated and compared. We modified the shell morphology from nanoparticles to nanorods, because nanorods can result in a reduced surface area of the shell such that the outer shell would have less contact with the corrosive electrolyte, resulting in improved electrochemical properties. Electron microscopy studies coupled with electron probe X-ray micro-analysis revealed the presence of a concentration gradient shell consisting of nanoparticles and nanorods before and after thermal lithiation at high temperature. Rietveld refinement of the X-ray diffraction data and the chemical analysis results showed no variations of the lattice parameters and chemical compositions of both produced CGCS particles except for the degree of cation mixing (or exchange) in Li and transition metal layers. As anticipated, the dense nanorods present in the shell gave rise to a high tap density (2.5 g cm(-3) ) with a reduced pore volume and surface area. Intimate contact among the nanorods is likely to improve the resulting electric conductivity. As a result, the CGCS Li[Ni0.60 Co0.15 Mn0.25 ]O2 with the nanorod shell retained approximately 85.5% of its initial capacity over 150 cycles in the range of 2.7-4.5 V at 60 °C. The charged electrode consisting of Li0.16 [Ni0.60 Co0.15 Mn0.25 ]O2 CGCS particles with the nanorod shell also displayed a main exothermic reaction at 279.4 °C releasing 751.7 J g(-1) of heat. Due to the presence of the nanorod shell in the CGCS particles, the electrochemical and thermal properties are substantially superior to those of the CGCS particles with the nanoparticle shell.