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Ceder group at MIT analyzes the limits and opportunities of phosphates as Li-ion cathode materials using a Materials Genome approach

Hautier
. Mean voltage in phosphates versus maximum gravimetric capacity achievable. Specific energy curves at 600 and 800 Wh/kg are drawn on the figure (blue dashed lines). The red dashed line indicates the upper voltage the authors considered safe against decomposition of the electrolyte. Different colors and markers have been used for different elements. Credit: ACS, Hautier et al. Click to enlarge.

Researchers at MIT, led by Prof. Gerbrand Ceder, have performed a high-throughput ab initio analysis of phosphates as Li-ion cathode materials, computing the voltage, capacity (gravimetric and volumetric), specific energy, energy density, stability, and safety of thousands of phosphate compounds. From the work, they have built a computational database of thousands of known and virtual phosphate materials.

The large database of calculated properties enables the researchers to draw important conclusions on the limits and opportunities for phosphates as cathode materials. Their paper is published in the ACS journal Chemistry of Materials.

Ab initio computations in the density functional theory (DFT) framework have been used for almost 20 years in the battery field to provide insight into the fundamental properties of electrode materials. Ab initio computations are nowadays accurate enough to understand and even predict many important battery properties (e.g., voltage, stability, safety, and lithium diffusion). The high scalability of computing furthermore offers the possibility to search for new cathode materials using a computational high-throughput approach by computing properties on thousands of potential battery materials. This approach can be used to screen and discover new or overlooked compounds...but also to analyze, using a large data set, the limits and factors that control electrochemical properties across an entire chemical class.

—Hautier et al.

Ceder leads the CEDER (Computational and Experimental Design of Emerging materials Research group) at MIT, and has been pushing for the realization of the “Materials Genome” project to bring advanced automated computing environments coupled to databases to materials design. Initial areas of application for the Materials Genome are in energy storage, photovoltaics and photocatalysis, thermoelectrics, and mercury adsorption from high temperature coal gasification.

In June, President Obama announced the Materials Genome Initiative, a more than $100-million effort to discover, develop, manufacture, and deploy advanced materials at twice the speed than is possible today, at a fraction of the cost. (Earlier post.) The Federal Materials Genome Initiative is one of the key steps in the Obama Administration’s Advanced Manufacturing Partnership (AMP).

For the paper on phosphates, Ceder and his group obtained their set of known compounds from the 2006 ICSD database. All known compounds containing lithium, phosphorus, oxygen, and a redox active metal were considered. Duplicate crystal structures were identified and removed.

In addition to known compounds from the ICSD, they generated “virtual” compounds by substituting ions in known ICSD compounds to form new possible compounds containing lithium, phosphorus, oxygen, and a redox active metal . This substitution process was driven by a data-mined substitution model they developed earlier.

To evaluate stability of the computed compounds, they also computed all competing phases known in the ICSD. In addition, a data-mined ternary oxide structure prediction algorithm has been used to propose any likely new compound candidate. In total, their resulting database contains 4,074 computed compounds containing lithium, a redox active element, phosphorus, and oxygen.

Among the many findings of the paper:

  • The maximum achievable gravimetric capacity for a phosphate is smaller than for an oxide because of the lower negative charge per unit wight for the (PO4)3- anion. The lower densities from phosphate compounds lead to on average 37% lower volumetric capacity compared to that of oxides. The higher voltage achievable in phosphates only slightly mitigates the impact on the energy density, which is on average 30% below that of oxides with the same capacity.

  • Statistically, V3+/V4+, V4+/V5+, Cr2+/Cr3+, Mn2+/Mn3+, Fe2+/Fe3+, Cu1+/Cu2+, Mo3+/Mo4+, Mo4+/Mo5+, Mo5+/Mo6+, Sb3+/Sb4+, and Sb4+/Sb5+ as the redox couples of interest for phosphate-based cathodes in current electrolyte technology (above 4.5 V).

    The inductive effect is observed in the data set with higher voltages identified for phosphates compared to oxides. This inductive effect makes several couples that are attractive in oxides, such as Mn3+/Mn4+, Fe3+/Fe4+, Cr3+/ Cr4+, Cu2+/Cu3+ Co2+/Co3+, Co3+/Co4+, Ni2+/Ni3+, and Ni3+/ Ni4+, too high in voltage for commercial electrolytes, even though they might be of interest with advanced electrolyte technology.

    Hautier et al. note that this statement is statistical, and some of those couples, if present in the right low voltage crystal structure, could show activity at voltages lower than 4.5 V. This is especially true for couples with an average voltage very close to the 4.5 V limit such as Mn3+/Mn4+, they said. Besides the inductive effect, the electrostatic and the P/O ratio also influence the voltage.

  • Their results challenge the common statement that olivine-based compounds and phosphates in general are intrinsically safer than oxides. They computed that thermodynamically, most phosphate compounds evolve oxygen at lower temperature than oxides for the same oxidation state of the active redox metal. Phosphates can, however, provide in most situations cathode materials with higher voltages than oxides for a similar thermal stability. They concluded that the reason that LiFePO4 is so stable against reduction by the electrolyte should at least in part be attributed to its low voltage.

    This finding is consistent with their computational and recent experimental evidence that indicates that the higher voltages olivine (LiMnPO4, LiCoPO4, and LiNiPO4) do not share the excellent thermal stability of LiFe- PO4, they said.

  • The only one-electron couples presenting acceptable safety combined with voltages in the 3–4.5 V window and that are lightweight are Fe2+/Fe3+, Mn2+/ Mn3+, and Cu1+/Cu2+. The two first couples have been extensively studied in olivine LiMPO4.

  • Because of the limitations on the gravimetric and volumetric capacity of phosphates, two-electron systems are of strong interest with this chemistry. Their analysis showed that few elements could provide two-electron activity in the 3–4.5 V voltage window.

    No M2+/M4+ redox couple was found to satisfy those requirements. Only molybdenum (Mo3+ to Mo6+) and vanadium (V3+ to V5+) show redox couples in the targeted voltage window with high enough achievable gravimetric capacities.

    Their compound prediction algorithm identified a novel two-electron vanadium compound, Li9V3(P2O7)3(PO4)2, to be very close to stability at zero K. This mixed ortho- and pyrophosphate, isostructural to Li9Fe3(P2O7)3(PO4)2, presents attractive theoretical specific energy (726 Wh/kg) and has been synthesized and tested electrochemically in their research group.

    The Sb3+/Sb5+ redox couple is also of interest, but the weight of antimony combined with the strong preference for different local environment for Sb3+ and Sb5+ (8-fold coordinated and octahedral) are detrimental.

  • Oxyphosphates are an attractive subclass of phosphates with their possible higher capacity because of their lower phosphorus content, while still benefiting from the inductive effect. Only a few oxyphosphates are known. Most of them exist in vanadium, molybdenum, and titanium chemistries.

    The lithium vanadium oxyphosphate Li-VOPO4 has been already well studied and shows good specific energy but poor rate capability. The two-electron redox couples available for vanadium and molybdenum combined with their tendency to form oxyphosphates call for more investigations of these chemistries, they said.

  • The data provided in their study work can be used to propose new chemistries to explore but also to suggest incremental modifications to known compounds.

  • For all properties investigated, except the voltage, LiFePO4 is close to the optimal of what could be expected from a one-electron phosphate. The 170 mAh/g capacity is the highest achievable (except for oxyphosphates). Olivines are among the most dense phosphate compounds with volumetric capacities around 590 mAh/cm3, and the safety of the delithiated iron olivine is extremely good for a material working at 3–5 V. Hence, unless one can increase the voltage of olivines without sacrificing other properties, not much improvement should be expected for one-electron phosphates.

All battery properties cannot be directly computed ab initio on a large scale, and our investigations only focused on some necessary but not sufficient conditions that new phosphate cathodes should meet. We believe the developments in accurate modeling of lithium diffusion and polaron migration will in the future provide the ability to perform the same kind of analysis on a larger set of properties such as ionic and electronic conductivity. Beside the difficulties in accurately modeling those properties, many challenges lies in the scaling up of their computations without human intervention in a high-throughput framework.

—Hautier et al.

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Comments

kelly

Seems to confirm present commercial battery composition, but important to see a 4000+ cathode material tested spectrum.

DaveD

"The red dashed line indicates the upper voltage the authors considered safe against decomposition of the electrolyte."

They are assuming current electrolytes. There is already proof that solid state electrolytes will exceed these voltage limits. I'm sure there will be other electrolytes that will do better as well.

But this is good work and helps to frame the expectations and limits of current and near term technology.

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