(As cycle life still needs to be improved for automotive applications (USABC long-term goals for EV batteries call for 1,000 cycles at 80% DOD and 10 years, earlier post), the advanced batteries with their attractive energy densities may emerge earlier in critical portable power applications. As an example, the military’s BB-2590 Li-ion battery used in a range of portable systems calls for a cycle life of ≥224 and ≥ 3 years.)

The silicon challenge. Silicon as an anode material for Li-ion batteries has attracted a great deal of R&D attention due to its much higher theoretical storage capacity than the graphitic anode materials commonly used in Li-ion batteries (LIB).

LIB capacity is limited in part by the intercalation of Li ions by the anode material—i.e., higher capacity batteries require anode materials that can accommodate more lithium ions. The theoretical capacity of the graphite anode is 372 mAh g^-1; the theoretical capacity of silicon is some 10 times that.

One of the significant challenges with silicon, however, is that it undergoes an enormous volume expansion when fully lithiated. These volumetric stresses associated with lithiation/delithiation cycles result in the material crumbling, and in battery failure after a relatively short number of cycles, especially when viewed from the perspective of automotive requirements.

Hence, a significant amount of effort and funding is flowing into ways to synthesize silicon-based materials that can exploit the potential capacity while delivering acceptable cycle life. Using nanostructures has been shown by many to be one of the promising approaches to providing tolerance to the extreme changes in volume with cycling. A challenge with nanostructured silicon materials is delivering the required functionality (energy density, cycle life, safety) at an acceptable manufacturing cost.

Envia Systems. Envia Systems was selected for a $4-million grant from ARPA-E in the Agency’s first round of solicitation in 2009 (earlier post) and received another $1 million from the California Energy Commission (CEC). (GM is also an investor, having put $7 million into the company. Earlier post.) Envia is targeting its high energy density Li-ion cells at plug-in hybrid and electric vehicles.

Envia Systems is developing large capacity pouch cells based on a novel high-voltage Manganese rich (HCMR) layered-layered Li₂MnO₃·LiMO₂ composite cathode with a Si-carbon anode and proprietary electrolyte. Having a high amount of manganese in the structure translates to high capacity, increased safety and low cost.

Envia’s HCMR cathode material is based on layered-layered cathode work licensed from Argonne National Laboratory (ANL). Envia has built on Argonne’s layered-layered chemistry to fine-tune the composition of Ni, Co, Mn and Li₂MnO₃. It innovated on particle morphology (particle size, shape, distribution, tap density & porosity) and also developed novel nanocoatings to enhance cycle life & safety.

The HCMR cathode materials offer capacity of 220-295 mAh/g, and power of >1200 W/kg; cycle life @ 80% DOD is more than 1,000.

Combined with a conventional graphite anode, the HCMR cathode would support a cell energy density of around 220-230 Wh/kg. Combined with the Si-C anode, it supports the heralded 400 Wh/kg density.

Envia’s highest capacity silicon-carbon anode. Click to enlarge.

With support from the ARPA-E grant, Envia has demonstrated silicon-carbon nanocomposite anodes with very high capacity (1,530 mAh g-¹) and promising cycle life. The material features nanopores with certain nanocoatings, said Dr. Sujeet Kumar, Envia Systems Co-Founder, President & CTO, in an interview with Green Car Congress at the recent ARPA-E Energy Inovation Summit (EIS). The silicon is embedded with the nanostructure. The approach is mundane and cost-efficient, he said, not exotic. The company is currently scaling up the material using a low-cost production process.

Envia has also developed an electrolyte that is stable up to a voltage of 5.2V (vs Li/Li⁺). In cyclic voltammetry studies of standard electrolytes, as the voltage window was opened from upper cut-off of 4.3V vs Li/Li⁺ the electrolytes showed an increase in oxidation currents. When the voltage window was further increased to voltages above 4.5V the oxidation currents increased significantly showing that the electrolytes were almost completely oxidized at these voltages. However, Envia’s High Voltage Electrolyte showed stability up to 5.2V without any rapid increase in the oxidation currents.

We made the announcement of 400 Wh/kg to increase the confidence of the electric vehicle community. —Atul Kapadia

ARPA-E tasked the Naval Service Warfare Center, Crane Division (NSWC Crane) Test & Evaluation Branch to perform Verification & Validation testing on two of the Envia pouch cells. The testing included verification of cell capacity and energy density at C/10 and C/3, 100% depth of discharge (DOD), as well as cell capacity and energy density at C/3, 80% DOD.

The test results from the prototype cells tested at Crane were in line with the results obtained from the manufacturer. The claims of 400 Wh/Kg were substantiated through the cycling tests performed at Crane.

Envia Systems expects it will be able to announce two commercial agreements this year, said Atul Kapadia, Chairman and CEO, in the EIS interview in which he also emphasized the importance of Envia’s focus on the automotive industry and its work with automotive partners.

It has taken us 5 years to get here; the 5 years can give us an insurmountable lead. We are a part of the [vehicle] product roadmap.

—Atul Kapadia

Nanosys. Advanced materials company Nanosys, Inc., the developer of a silicon-graphite anode technology (SiNANOde), was awarded a $4.8 million grant from the DOE in August 2011 for the development of Li-ion cells leveraging high voltage composite cathode materials and silicon-based anodes. (Earlier post.)

In this project, Nanosys and LG Chem Power are developing a 700~1000 mAh g^-1 Si anode (SiNANOde) with a target cycle-life of >800, and an eventual goal of achieving an energy density of 1,600 mAh g^-1 at the end of the program. Combined with an innovative 255 mAh g^-1 cathode (Mn-rich) and unique large format cell the Si anode is expected to enable a cell with 350 Wh/kg at a cell level cost target of <$150/kWh.

This project will also use a layered-layered cathode material licensed from Argonne (which LG Chem Power and GM have licensed from Argonne (earlier post). With this technology LGCP has demonstrated a cathode specific energy of 255 mAh g^-1.

Nanosys grows silicon nanowires on graphite particles for its anode solution (SiNANOde); the intention is to provide the material as powder to battery manufacturers who can then integrate the powder into their existing production processes, Vijendra J (VJ) Sahi, Vice President of Corporate Development, told Green Car Congress in an interview at the ARPA-E EIS.

To achieve the goals of the DOE project, Nanosys will need to improve SiNANOde capacity from 650 mAh g^-1 to 700~1000 mAh g^-1 in Phase I and subsequently to 1,600 mAh g^-1. The Si content in the powder will need to be increased to 40%. Graphite particle size and morphology will be further optimized to achieve this goal.

Achieve increased endurance of cycle-life will need to increase from 220 to >800. To achieve this requires surface modification of the Si nanowire anode for improved stability and SEI formation, as well as optimization of the electrolyte and binder chemistry will be optimized. The company has so far achieved 700 cycles with 70% capacity retention at 100% DOD (depth of discharge), and 300 cycles with 80% retention, Sahi said.

Argonne National Laboratory. Argonne, with funding from ARPA-E, has developed a process to synthesize silicon-graphene composites as anode materials for Li-ion batteries. When combined with Argonne’s high energy composite cathode material, the resulting cell will offer more than 340 Wh/kg, according to Dr. Junbing Yang, one of the inventors of the Si-graphene material. Argonne has filed two patent applications on this technology, which it also was highlighting at the EIS.

Argonne’s gas phase deposition approach embeds nano-scale silicon particles into the graphene layers, which is the key for longer cycle life, Dr. Yang says.

California Lithium Battery. California Lithium Battery has entered into a Work for Others (WFO) program with Argonne National Laboratory to commercialize an advanced Li-ion battery combining ANL’s Si-graphene anode materials with other advanced battery materials into a Very Large Format (400 Ah) prismatic cell. The primary application for this GEN3 Li-ion battery is for grid-scale storage and electric vehicles, the company said. CalBattery has an option for exclusive and non-exclusive rights to the ANL Si-graphene process.

CalBattery, a joint venture between Ionex Energy Storage Systems and CALib Power, was a runner-up in the DOE “America’s Next Top Energy Innovator” Challenge. (Earlier post.) The company says it plans to start manufacturing a line of lowest cost per watt cells for these markets in the US starting in 2014.

Resources

• Junbing Yang, Wenquan Lu, Nanotube composite anode materials suitable for lithium ion battery applications, US Patent App. 12/938,638; Publication number: US 2011/0104551 A1; Filing date: 03 Nov 2010

• Khalil Amine, Junbing Yang, Ali Abouimrane and Jianguo Ren, Composite materials for battery applications, US patent App. 13/100,579, filed on 04 May 2011

• DOE Vehicle Technologies Program FY 2011 Progress Report for Energy Storage R&D