Fujitsu Laboratories Ltd. has successfully developed a high-voltage cathode material for lithium iron phosphate rechargeable batteries. Using a proprietary materials design technology as well as a technology that precisely controls the composition of raw materials and the formation process of materials, Fujitsu Laboratories has successfully synthesized lithium iron pyrophosphate (Li5.33Fe5.33(P2O7)4). This phosphate-based material has a voltage of 3.8 V, comparable to that of existing cobalt-based materials.
The new Fe-based cathode has a potential for Fe2+/3+ redox couple approaching to 3.8—the highest among those of all Fe-based phosphate materials and pyrophosphate materials reported so far, including LiFePO4, Li3Fe2(PO4)3, LiFeP2O7, Li2FeP2O7 and LiFe1.5P2O7. Fujitsu engineers is announcing details on the material at the 231st ECS Meeting in New Orleans this week.
|Crystal structure of the new material.|
Currently, lithium-ion batteries are widely used as high-performance rechargeable batteries. However, there are concerns about insufficient supply and rising costs, as the batteries’ cathode materials contain the rare metal cobalt, such as lithium cobalt oxide (LiCoO2). Large volumes of lithium-ion batteries will be required in the future for electric vehicles in order to achieve a low-carbon society that does not rely on fossil fuels and emit greenhouse gasses; accordingly, there has been a great deal of interest in developing materials that use earth-abundant iron as a constituent element in place of cobalt.
Current lithium-ion batteries using iron-based materials do not reach the energy density of those using cobalt-based materials. Energy density is expressed as a product of capacity density and voltage. Iron-based materials with voltage of 2.8 V to 3.5 V thus could not compete with cobalt-based materials whose voltage ranged from 3.75 V to 4.1 V. It is known that the voltage of cathode materials can change depending on the arrangement of atoms in the crystal structure, which created issues in the development of new iron-based materials with high voltage.
By analyzing the correlation between the crystal structure of iron-based materials and their electrochemical characteristics, Fujitsu Laboratories discovered new factors in improving the voltage of iron-based materials.
The voltage of cathode materials is significantly influenced by the coordination of elements such as iron and oxygen in the crystal. By analyzing the interrelationship between the crystal structure of a material and its electrochemical characteristics, Fujitsu Laboratories discovered new factors in improving the voltage of iron-based cathode materials. The researchers found that a distorted arrangement of oxygen atoms around iron atoms is one of critical factors for the high voltage.
Specifically, they suggested that the high potential of 3.80 V is probably related to the crystal structure with the FeO6 edge-sharing chains, in which the distance between neighboring Fe atoms is relatively small (3.22 Å at the smallest). This feature makes large Fe-Fe repulsion energy in the charged state with Fe3+, resulting in large difference in free energy between the charged and the initial states, which determines the redox potential.
The cathode material was synthesized via a solid-state method. Stoichiometric amounts of precursors, Li2CO3, FeC2O4 and (NH4)2HPO4, were wet-blended in acetone. After evaporation of the solvent, the resulting mixture was sintered at 500-650 °C for 12 hours in Ar atmosphere to obtain the cathode material.
Synchrotron X-ray powder diffraction patterns of the cathode material were obtained using a wavelength of 0.9996 Å. Rietveld refinement was carried out with RIETAN-FP program.
Half-cell assembling was conducted in a dry room (dew point: <-70°C). The mixture of the cathode materials, Ketjen Black and polyvinylidene difluoride in a ratio of 85:10:5 wt% was dispersed in N-methylpyrrolidone. The resulting paste was coated on Al sheet, and then dried in vacuum at 40 °C to evaporate the solvent. The Al sheet was cut out in disks (φ =16 mm), pressed under 9.5 MPa, and then dried in vacuum at 120°C overnight. 2032-type cells were assembled using the cathode sheets mentioned above as a positive electrode, Li metal disks (φ =16 mm) as a negative electrode, 1M LiPF6solution in a 3:7 v/v mixture of ethylene carbonate/dimethyl carbonate as the electrolyte, and polypropylene separator (φ =18 mm). Galvanostatic charge and discharge measurements were carried out in CC mode (6.95 mA per 1 g of the cathode material). The voltage range was set to 2.0-4.5 V.
The charge capacity of the prototype coin battery is about 105 mAh/g—approximately 75% of the theoretical value of 139 mAh/g (Li5.33Fe5.33(P2O7)4), or the actual value of 137 mAh/g (LiCoO2). Through continued analysis, Fujitsu Laboratories plans to further improve such figures.
Based on this cathode development, Fujitsu Laboratories will work to design a crystal structure that can maintain a voltage on par with cobalt-based materials for longer periods. The electrode can also be used as a low-cost cathode material in safe, solid-state rechargeable batteries.
Tomochika Kurita, Jun-ichi Iwata, Tamotsu Yamamoto, Shintaro Sato (2017) “A New Lithium Iron Pyrophosphate Material with Abnormally High Voltage Approaching to 3.8 V” ECS 213 Nº 413