Development of Li-S technology at OXIS Energy

Sébastien Desilani, Senior Scientist

September 2016, Nice

OXIS Company Background

$70 million investment

Expanding rapidly:

3 fold increase in the number of employees since 2012, 60 today

Cutting edge R&D facilities (i.e. second largest high specification dry room in Europe)

Strong patent portfolio protecting IP (79 patents granted, 97 pending, encompassing 27 families)

OXIS have been working on Li-S since 2005 at CulhamScience Centre (Oxfordshire, UK)

2

Li-S Battery Operating Principles

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 666157

Li

Cu

rren

tco

llect

or

Cu

rren

tco

llect

or

Li+

Li+

Li+

Li+

Li+

Li+

(-) (+)

Sep

arat

or

+-

Discharge

Load / Charger

S8

(-) : 16 Li° → 16 Li+ + 16 e-

(+) : S8 + 16 e- → 8 S2-

16 Li° + S8 → 8 Li2S

Elemental sulfur

Conductivecarbon

Binder

Average voltage: 2.1 V (vs. 3.7 V of Li-ion)

Sulfur electrode specific capacity:1675 mAh g-1 (vs. 170 mAh g-1 of LiFePO4)

Complex working mechanism:with intermediate species (soluble Li2Sx)

Theoretical gravimetric and volumetric energy: 2500 Wh kg-1 and 2800 Wh L-1, respectively

4

Li-S: A complex electrochemical system

Li2S

Li2Sx2-

Li2Sx•

Li2Sx2-

Li2Sx2-

Li2Sx2-

Li2Sx2-

e-

Li+

Diffusion of polysulfides from electrode surface

Electrolyte reaction with polysulfide radicals

Electrolyte reaction with anode surface

Anode stripping/plating during charge/discharge

SEI layer cracking

Electrolyte

Li2Sx2-

reduction at anode

Li2Sx2-

Li2SOx

Irreversible oxidation

ANODE SEPARATOR CATHODE

Current collector corrosion

Carbon/binder skeleton collapse

Li2S

Poorly soluble Li2S

Li+

Li+

Li+

Growth of mossy/dendritic lithium

Electrolyte

Li2Sx2-

S8

5

Li-S cell development at Oxis Energy

Q2 2015: 10Ah CellEnergy Storage/LEV’s160Wh/kg

2010: 500 mAhpouch cell< 100 Wh/kg

ULT

RA

LIG

HT

LON

G L

IFE

2013-2014: Ultra lightR&D prototype2 Ah ; 220-240 Wh/kg

2016 Target:~15Ah cell Aerospace/Defence400 Wh/kg, 100 cycles

2016 Target : 20Ah cellEnergy Storage/LEV 250Wh/kg

Q3 2016: Ultra light UAV market18 Ah ; 400 Wh/kg

OXIS Long Life technology

6

0

100

200

300

400

500

600

0 250 500 750 1000

Cap

acit

y, m

Ah

/g(S

)

Cycle Number

Charge

Discharge

OXIS Longlife electrolyte enables lithium metal to be reversibly plated and exhibits high safety

Reducing the DOD to 80% leads to >1000 cycles due to polysulfide shuttle control

Reducing the DOD to 30% leads to virtually no fade due to low Li utilisation and low volume change in the cathode

Patent : WO 2014/155069

Development of High Energy density Li-S

Cathode: Increasing the surface capacity

• Highly conductive carbon sulfur composite allow increased surface capacity while maintaining high discharge capacity

• Surface capacity > 4mAh/cm² already leads to Energy density >400Wh/Kg

0

100

200

300

400

500

600

0 5 10 15 20 25

Ener

gy d

ensi

ty i

n W

h/K

g

Cathode surface capacity in mAh/cm²

Impact of cathode surface capacity onto the energy density of the cell

Cathode: Importance of the binder

• Functional binders can anchor polysulfides and prevent cathode passivation by Li2S

Binder 1 after cycling : little cathode clogging

Binder 2 after cycling : cathode passivated

Lithium Metal Anode: The Challenges

Thermodynamically unstable against any kind of organic electrolyte

Low coulombic efficiency

• Electrolyte and lithium consumption Drying of a cell leading to failure

• Dendritic growth and mossy deposition Destroyed integrity of the anode, failure with tab disconnection, safety issues

Lithium metal

(a)

Lithium anode after 200 cycles: (a) SEM and (b) photographic image

(b)

Company Confidential

50 µm

10

Coatings: increasing the reversibility of lithium plating

Thin protective coatings lead to better lithium deposition and hence lower the rate of electrolyte depletion

100 Cycles 100 Cycles

Electrolyte: Increasing Cycle life

Electrolyte is carefully selected for:

• Tailored polysulfides solubility

• Low viscosity for lithium plating

• High lithium mobility

Cycle life with OXIS’ electrolyte is doubled over the commonly used DME:Diox

Electrolyte: Increasing discharge capacity with functional additive

Electrolyte is critical component for S8 utilisation

Development of new electrolyte for higher energy density:

• Additive increase Li2S2 and Li2S solubility up to 20% extra capacity

• Lower the charge overpotential

Electrolyte: Increasing Power of Li-S cells

Development of new electrolyte for higher power capability:

• Higher conductivity

• Target: 2C continuous for PHEV application

• Tailored PS solubility

• Maintain 75% of C/5 capacity at 1C

State of Available Power in Li-S

Peak power is strongly dependent on the state of charge

Typical behaviour of the 300Wh/Kg OXIS cells

• 300 Wh kg-1 + 70 cycles to 80% BoL capacity at 0.1 C and 30°C

• Cell dimensions 76 x 142 x 12 mm

• 13 Ah at 0.1 C and 30°C

• Little temperature change

• Thickness variation is very uniform state of charge can be easily determined by thickness measurement as well as SOH

• Ideal for drones

Development of cells > 400 Wh kg-1

Latest OXIS development (under characterisation)

• Higher surface capacity cathode

• Lighter anode

• Improvements to electrolyte to gain more mAh g(S)-1

• Currently under full characterisation

Next step: Anode protection implementation for >100cycles at 400Wh/Kg

145 mm

80 mm11 mm

BMS Development: SOC Estimation

Test Model Predict –Measure-Feedback

First for Li-S SOC estimation - Starts from initial SOC

- Whole battery capacity is not needed- Fast enough for real time application

- Limited use case development on-going

• Developing Fast Parametrisation Algorithms

• Reasonable predictions

• Aging mechanisms and self discharge to be included Patent pending

Innovate UK funded project

Summary

• Today Li-S can provide 400Wh/Kg compared to 240Wh/Kg for the best Li-ion cells

• Carbon sulfur composites enable high surface capacity cathode for high gravimetric energy cells

• Anode protection is key for cycle life and volumetric energy

• Electrolyte development enable decent power performances and high S8 utilization

• SOC can be accurately estimated from various parameters

www.oxisenergy.com

Thank you for listening

Acknowledgment: OXIS’ team