air battery

Journal Pre-proof Environmental and economical assessment for a sustainable Zn/air battery F. Santos, A. Urbina, J. Abad, R. López, C. Toledo, A.J. Fernández Romero PII:

S0045-6535(20)30466-5

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126273

Reference:

CHEM 126273

To appear in:

ECSN

Received Date: 3 October 2019 Revised Date:

10 February 2020

Accepted Date: 17 February 2020

Please cite this article as: Santos, F., Urbina, A., Abad, J., López, R., Toledo, C., Fernández Romero, A.J., Environmental and economical assessment for a sustainable Zn/air battery, Chemosphere (2020), doi: https://doi.org/10.1016/j.chemosphere.2020.126273. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Graphical Abstract

1

Environmental and Economical Assessment for a Sustainable

2

Zn/Air battery

3 4 5 6 7

F. Santos*1, A. Urbina*2, J. Abad1, R. López2, C. Toledo2, A.J. Fernández Romero1

8

Abstract

9

Metal/Air batteries are being developed and soon could become competitive with other

10

battery technologies already in the market, such as Li-ion battery. The main problem to

11

be addressed is the cyclability, although some progress has been recently achieved. A

12

Life Cycle Assessment (LCA) of the manufacturing process of a Zn/Air battery is

13

presented in this article, including raw extraction and process of materials and battery

14

assembly at laboratory scale (cradle to gate approach). The results indicate that Zn/Air

15

battery can be fabricated with low environmental impacts in most categories and only

16

four deserve attention (still being low impacts), such as Human Toxicity (cancer and

17

non-cancer), Freshwater Ecotoxicity and Resource Depletion (the later one depending

18

mainly on Zn use, which is not a critical material, but has a strong impact on this

19

category). Cathode fabrication arises as the subassembly with higher impacts, followed

20

by membrane, then anode and finally electrolyte. An economic cost calculation

21

indicates that if cyclability of Zn/Air batteries is achieved, they can become competitive

22

with other technologies already in the market.

23 24 25 26

*Corresponding authors: Antonio Urbina (e-mail: [email protected]) and Florencio Santos (e-mail: [email protected])

27

Keywords: Energy storage, Zn/Air battery, Life cycle assessment, Environmental impact.

1

Grupo de Materiales Avanzados para la Producción y Almacenamiento de Energía, Univ. Politécnica de Cartagena, Campus de Alfonso XIII, Cartagena, Spain. 2 Departamento de Electrónica, Univ. Politécnica de Cartagena, Plaza del Hospital 1, Cartagena, Spain.

1

28

1. Introduction. Introduction.

29

The International Energy Agency predicts that primary energy from renewable sources

30

will surpass fossil fuel generation in 2050, when total primary energy demand will be

31

more than 700 Exajoules, half of it provided by renewable sources (IRENA, 2019). The

32

transition towards an energy system with high penetration of renewable energy,

33

specially in electricity generation, demands the use of energy storage (Kondoh et al.,

34

2000). The intermittency of energy generation and the mismatch in real time between

35

generation and consumption for renewable energies requires energy storage at different

36

scales for mechanical, thermal or electrical energy (Alotto et al., 2014). Nowadays,

37

batteries are the most reliable energy storage systems for different applications, such as

38

portable gadgets, electric cars, photovoltaic systems or grid stabilization, thus pointing

39

to a future upscaling of production according to the predicted demand for energy

40

storage(Armand and Tarascon, 2008; Hesse et al., 2017).

41

For the fabrication of any kind of battery a large amount of raw material and energy are

42

consumed during the process, waste and disposal also generates an important

43

environmental impact (Dehghani-Sanij et al., 2019). All solutions for energy storage

44

provided by batteries should be sustainable from an environmental and an economical

45

point of view and methodologies to evaluate its environmental impact during

46

fabrication, use and eventually recycling and/or landfilling phases should be carried

47

out(Van Den Bossche et al., 2005). Lead-based, alkaline or lithium-ion batteries have

48

well stablished operational parameters and competitive capital costs, with round-trip

49

efficiencies around 85% and operating cycles above 3000 (Li-ion above

50

7000)(Zackrisson et al., 2016). Special attention deserves the redox-flow batteries, since

51

they are envisaged as the best option for grid energy storage in peak times; several

2

52

technological

options

are

being

studied:

53

iron/chromium; Fe-EDTA/bromine, Zinc/Cerium, etc… with potential for the redox

54

couple in the range of 1.2V to 3.4V (Weber et al., 2011, 2018); recent advances in

55

redox-flow batteries using low cost carbon polymer composites and graphene based

56

nanoparticles have extended their lifetime (Chakrabarti et al., 2014; Lobato et al., 2017)

57

and accelerated degradation charge-discharge studies have shown that bench-scale

58

vanadium redox flow batteries (VRFB) can be adequate for storage of solar photovoltaic

59

electricity (López-Vizcaíno et al., 2017) and wind electricity (Mena et al., 2018). For a

60

summary of typical parameters of some current and emerging battery technologies see

61

Table S1 in the supplementary information.

62

However, innovative technologies such as Li/Air and Zn/Air, with an energy density

63

theoretically ten times higher, are being widely investigated; the environmental impact

64

for its production could be reduced by an amount between 4 and 9 times when

65

compared with conventional Li-ion and by recycling, up to 30% of production related

66

environmental impact could potentially be avoided (Fu et al., 2017; Zackrisson et al.,

67

2016). An important advantage of Zn/Air is the stability of fabrication components

68

towards moisture, contrary to Li/Air which requires inert atmosphere for handling the

69

materials, thus making the Zn/Air battery potentially most suitable for cheap massive

70

industrial production(Lee et al., 2011).

71

Nevertheless, it is difficult to compare the environmental impact of well stablished and

72

emerging technologies since their use-phase (specially cycling and lifetime) is very

73

different and it makes difficult to propose a functional unit for a comparative LCA. In

74

this article the adequate LCA methodology that should be applied to compare

75

conventional batteries with innovative approaches such as Metal/Air batteries is

3

all-vanadium;

vanadium/bromine;

76

discussed, with special focus on a laboratory scale production of Zn/Air batteries; it is,

77

therefore, a prospective work, which sets impact values that could be reduced if an up-

78

scaled industrial process is considered.

79

Metal/Air batteries have attracted much attention recently, due to the high capacity and

80

energy densities that they can develop. Nowadays, primary Zn/Air is the only Metal/Air

81

battery with a real commercial application. Contrarily, rechargeable Metal/Air batteries

82

have not been sufficiently improved to reach a commercial level. However, several

83

research groups have focused their investigations on new materials used in this type of

84

batteries with the aim to reach a Metal/Air rechargeable battery. Li/Air, Na/Air, Al/Air

85

and Zn/Air batteries are the main systems that are under investigation.

86

Use of Zn as negative electrode has many advantages, such as its low cost, abundance

87

of Zn in the natural medium or the availability of use aqueous-based electrolytes(Li and

88

Dai, 2014; Santos et al., 2018; Zhang et al., 2015). Recently, important progress for

89

secondary Zn/Air and other Zn-based batteries has been reached (Mainar et al., 2016,

90

2018b; Pei et al., 2014). Yan et al. have published results demonstrating up to 95%

91

capacity retention after 4000 cycles for a zinc hybrid cell using doped LiMn2O4 as a

92

positive electrode(Yan et al., 2012). Besides, use of a non-aqueous electrolyte,

93

containing PC and fluor-based salt, in a secondary Zn-based battery provided more than

94

1700 cycles at 99.8% efficiency(Guerfi et al., 2014); B. Bugnet et al. developed a Ni/Zn

95

battery with 800 to 1500 cycles based on TiN ceramic conductor (Bugnet, 2014).

96

On the other hand, different bi-functional Air electrodes have been prepared to be used

97

in Zn/Air batteries and a high number of charge/discharge cycles has been reached(Pei

98

et al., 2014). Thus, Pan et al. developed a new type of flow Zn/Air battery with nano-

99

structured Ni(OH)2 and MnO2–NaBiO3 as bi-functional catalysts, displaying 1.32 V

4

100

during the discharging process, with an average coulombic efficiency of 97.4% and an

101

energy efficiency of 72.2% after 150 cycles (Pan et al., 2009). Amendola et al.

102

developed a tri-electrodes rechargeable Zn/Air battery arranged in a horizontal

103

orientation, which was not degraded after 2700 cycles(Amendola et al., 2012).

104

Additionally, a mesoporous Co3O4 NW array as a highly active bifunctional catalyst for

105

both oxygen reduction and evolution reactions was proposed as an advanced Air

106

electrode. Furthermore, 1500 pulse cycles are demonstrated before degradation,

107

exhibiting excellent rechargeability: after cycling of 600 h, charge and discharge

108

potential retentions of 97% and 94% were obtained, respectively (Lee et al., 2014).

109

These new advances in Zn/Air batteries allow us to be optimistic about reaching

110

acceptable batteries in the next years. Furthermore, the aim of this work is to highlight

111

environmental hotspots linked to the development of a reversible Zn/Air battery.

112

With respect to the batteries manufacture, production of raw materials by mining

113

industry creates important environmental impacts. A recent review(Dehghani-Sanij et

114

al., 2019) indicates that 85% of lead production worldwide is used in the fabrication of

115

lead-acid batteries according to the International Lead Association (“Lead Uses -

116

Statistics < Lead Facts | ILA - International Lead Association Website,” n.d.), while in

117

2017 already 45% of Li production was devoted to the fabrication of Li-ion batteries,

118

similarly 50% of cobalt and 10% of graphite production worldwide is used in battery

119

electrodes. Graphite has been declared a strategic material by the European Union

120

(European Commission, 2017, 2014). Toxicity of lead, although it is efficiently recycled

121

(more than 95%), probably will slowly reduce the production of lead acid batteries, in

122

spite of their low cost and good features. The environmental burdens of manufacture of

123

the Li-ion battery is dominated by the production of the negative and positive electrodes

5

124

and the battery pack (see Table S2) and are considered as the bench-mark for

125

comparison for Li/Air or Zn/Air (Notter et al., 2010). Energy consumption for current

126

Li-ion battery production is from 350 to 650 MJ/kWh, which brings GHG emissions to

127

figures between 120 and 250 kg CO2-eq/kWh(Posada et al., 2017; Romare and Dahllöf,

128

2017). For Li/Air battery, Zacrkisson et al. carried out a detailed LCA which calculated

129

a climate change impact of 1100 kg CO2-eq/kWh of energy delivered, considering only

130

the production phase of their study for the STABLE Li/Air battery prototype; the total

131

impact including use and end of life phases is 1299 kg CO2-eq/kWh, thus showing that

132

the higher impacts comes from the production phase(Zackrisson et al., 2016; Zhao and

133

You, 2019). For 1 MJ of energy storage capacity the impacts for Li-ion battery in

134

several categories are: Climate change 17–27 kg CO2-eq; Human toxicity 3-5 kg 1.4-

135

DB-eq; Metal depletion 28-44 kg Fe-eq, and Fossil depletion 2.2-3.4 kg oil-eq.

136

(McManus, 2012). Considering several studies, the average results for 1 kWh of energy

137

storage capacity in Li-ion batteries, the cumulative energy demand (CED) for

138

production is 328 kWh and 110 kg CO2-eq of greenhouse gas emissions (GHG) (Peters

139

et al., 2017). When detailed information about cyclability and lifetime of the battery are

140

available, the results for 1 kWh of electricity provided over the entire life cycle of a

141

battery, the CED is reduced to 26 kWh and consequently the GHG emissions are

142

reduced to 74 kg CO2-eq (Peters et al., 2017). The changing conditions with time for the

143

use of any energy storage system must also be taken into account for the LCA which

144

evaluates service-based functional units for a service extended in time, as it was applied

145

to Li-ion battery LCA (Elzein et al., 2019; Sun et al., 2016; Zhao and You, 2019).

146

Recycling of components to recover Li of Li-ion batteries could reduce environmental

147

impacts up to 30% (Zackrisson et al., 2016), but at present there is almost no industry

6

148

dedicated to the recycling of lithium traction batteries since the economic return is very

149

low (Wang et al., 2014); on the contrary, there is detailed information about Zn waste

150

recovery (Ng et al., 2016).

151

152

2. Materials and methods. methods.

153

The LCA methodology is widely applied to environmental impact analysis of products

154

and services in different impact categories. When applied to energy generation or

155

storage systems, climate change mitigation potential is the most widely used category

156

and the standard LCA methodology is often complemented with calculations of

157

embedded energy, energy pay-back time (or similarly CO2-eq embedded and CO2-eq

158

pay-back time). LCA has been standardized by the International Standards Organization

159

(ISO) in the ISO-14040 series (“ISO - ISO 14040:2006 - Environmental management

160

— Life cycle assessment — Principles and framework.Technical Committee : ISO/TC

161

207/SC 5,” 2006)

162

In order to define a functional unit based on the service provided, in this case the

163

amount of electricity stored and delivered in the battery throughout its lifetime, it is

164

necessary to clarify the limits for depth of discharge (DOD) that each kind of battery

165

considers for safe operation (usually around 80%) at which nominal lifespan is defined.

166

The lifespan is therefore the number of cycles for which cell capacity does not fall

167

below the specified limit that can be the DOD or lower for optimal cycling. An

168

additional difficulty for a functional unit based on service for batteries is that the limit

169

used to define the lifespan decreases with the time. A complete LCA study for batteries

170

should comprise at least three phases: production phase, where raw materials or

7

171

materials from recycling input should be considered; use phase for the different

172

applications (such as electric vehicles or photovoltaic systems) including maintenance

173

and end of life phase where final collection, disposal or recycling of the used battery is

174

carried out. In each stage, besides material, inputs of energy and gas emissions must be

175

taken into consideration, as well as other emissions. Each of the main phases may be

176

subdivided in stages depending on the scope of the LCA. Usually the selected functional

177

unit of the LCA is product-based when LCA focuses on production phase, or service-

178

based when LCA includes also the use phase. For batteries, it is common practice to use

179

two Functional Units, 1kg of battery and 1kWh of stored energy, sometimes extended to

180

1kWh of “lifetime” energy storage, when an average of all cycles capacity until end of

181

life is considered.

182

2.1. 2.1. LCA methodology for the Zn/Air battery. battery.

183

In its actual development level, rechargeable Zn/Air batteries present a lower cyclability

184

than Li-ion ones. Therefore, for a fair LCA of this kind of battery, the functional unit of

185

choice must be the nominal capacity of the battery and not the energy stored and

186

delivered throughout its lifetime. Thus, in the case of Zn/Air batteries, one single cycle

187

should be considered one single cycle and therefore LCA comparison with other battery

188

technology should be made at the end of the production phase or after one single

189

discharge. Promising results have demonstrated acceptable cyclability of Zn/Air

190

batteries as mentioned in the introduction(Mainar et al., 2018a), and thus, if cyclability

191

is improved for Zn/Air batteries or simulations are considered, then it can be treated as a

192

secondary battery and an easier comparison can be carried out. The LCA applied to

193

Zn/Air batteries in this work includes the following steps:

8

194

•

Goal and Scope: the cradle to gate scope is chosen for this study as

195

indicated in Figure 1, where the boundary is indicated. Recycling issues will

196

be commented but are not included in the LCA calculations.

197

•

Functional unit (FU): for this study the FU is stablished as 1 kg of

198

manufactured battery. The service phase of the batteries, which should be

199

taken into consideration for a service-based FU throughout the lifetime of

200

the battery is not considered, since number of cycles, optimum DOD and

201

therefore lifespan are parameters still to be optimized for the Zn/Air battery.

202

Some assumptions have been taken for the economical comparison of

203

Levelized Cost of Electricity (LCOE) provided by a hypothetical secondary

204

Zn/Air battery. When the final application is an electric vehicle, one

205

kilometer of displacement is often taken as service FU (Notter et al., 2010;

206

Zackrisson et al., 2016), then a well stablished number of cycles and lifespan

207

of the battery is required to compare the FU based on product process with

208

the FU based on delivered service. Comparison of FU for a cradle to gate

209

approach with a FU which includes service phase when applied to electric

210

vehicles is still under discussion since the overall efficiency and energy

211

losses of the tracking system depends on different powertrain configurations

212

and not only on battery properties(Nordelöf et al., 2014).

213

9

214 215 216

Figure 1. Scope and boundary of the LCA study for Zn/Air batteries: production phase (including recycling within this stage). Use phase and final recycling and disposal is not included.

217 218

•

Life Cycle Inventory (LCI): it involves the compilation and the

219

quantification of inputs and outputs of a given product system throughout its

220

life cycle or for a single process. In this case, a single process for the battery

221

fabrication is considered and several inventories have been carried out: the

222

material inventory, which is the collection of all material flows in the

223

production process for the FU; the energy inventory, which includes both the

224

energy embedded in the input materials for the manufacturing process and

225

the energy consumed during the process itself and finally, the emissions

226

inventory, which includes the releases to soil, water, and air generated during

227

the entire life cycle, but in this case is limited to the production phase.

228

•

Life Cycle Impact Assessment (LCIA): The LCIA identifies and evaluates

229

the amount and the significance of the potential environmental impacts

230

arising from the LCI obtained in the previous stage. In order to facilitate this

231

assessment, the inputs and outputs obtained from the LCI are classified and

10

232

are related to some environmental indicators, for example, climate change,

233

ozone depletion, human toxicity, etc., which are presented as impact

234

categories. From a selection of these categories a global index is often

235

stablished and presented to allow for a cross-field comparison between

236

different technologies. For the study of Zn/Air batteries two LCA methods

237

have been used in this article for impact analysis: mainly ILCD

238

Midpoint+V1.1 with equal weighting +CED, which is a method developed

239

by the European Commission Joint Research Center and uses equal

240

weighting to all categories which makes comparison with other methods

241

straightforward; for additional calculations ReCiPe 2016 Endpoint(H)V.1.02

242

(Damage Assessment) has been used because it enables a simple and

243

standard grouping of categories with focus on ecosystems damage and

244

resource depletion (shown in supplementary information).

245

The software SimaPro which provides access to the EcoInvent Swiss database have

246

been used to compile the data and to calculate the impacts of this LCA study (SimaPro

247

8.4.0 and EcoInvent 3.4, 2019).

248 249

2.2. Fabrication of Zn/Air batteries

250

Traditionally, Zn/Air batteries have a high energy density, but are not rechargeable due

251

to different problems, as it has been stated in the introduction. Nevertheless, Zn/Air

252

batteries are considered as the most promising candidate to compete with Li-ion

253

batteries, specially for its application in electric vehicles(Mainar et al., 2018a). Besides,

254

Zn rechargeability has been improved and nanoporous carbon fiber with or without

255

metal oxides films have been successfully used as positive electrodes in Zn/Air

11

256

batteries, thus opening an easier upscaling towards industrial production of rechargeable

257

Zn/Air batteries and reducing the environmental impact because the reduced use of

258

metallic catalyst such as Pt, Ir or Ru (Liu et al., 2016).

259

The basic structure of a Zn/Air battery is shown in Figure S1: The Zn/Air battery is

260

composed of Zn powder, as negative electrode, a PVA-KOH hydrogel polymer

261

electrolyte and a carbon black-based positive electrode including MnO2 as catalyst

262

material. Table 1 shows all materials used in the battery fabrication. Goodfellow and an

263

amount of 0.5 gr was used in the negative electrode supplied Zn powder (purity 98.8

264

%). PVA-KOH gel polymer electrolytes were synthesized as it was described

265

previously(Santos et al., 2019). Basically, 4 g of PVA were dissolved in deionized water

266

under stirring for two hours and maintaining the temperature below at 90 °C. When it

267

was at ambient temperature, 30 ml of KOH 6M was dropwise added maintaining the

268

stirring. The resulting liquid was then poured into a Petri dish and let it to cast. For this

269

amount, 22 specimens of 12 mm diameter were obtained. PVA-KOH soaked

270

membranes were prepared starting from samples of PVA-KOH gels dried for 10 days,

271

which subsequently were immersed in KOH 12M for 24. This procedure provided the

272

entrance of additional water molecules and KOH inside the membrane, increasing the

273

membrane weight (swelling ratio was 34±2 %). Moreover, conductivity values of 0.34

274

Scm-1 at 20 ºC were obtained for these membranes (Santos et al., 2019). PVA

275

MOWIOL 18-88 (MW 130.000 and KOH (85%) were obtained from Sigma-Aldrich.

276

Besides, MilliporeTM water with resistivity of >18 MΩcm was always used.

277

A scheme of the positive electrodes is shown in Figure S2. Catalyst layer was

278

prepared mixing MnO2, carbon-black and PVDF in minimum quantity of THF. The

279

slurry was stirred at room temperature for 1 hour. After that, the slurry was dried in the

12

280

oven at 80 ºC for 1 hour and finally was compacted at a pressure of 10 tons cm-2.

281

Finally, the resulting disc was press with the other components.

282

3. Results and discussion: LCA for Zn/Air batteries

283

The process for the fabrication of the Zn-air battery includes the materials and solvents

284

in the Life Cycle Inventory provided by Table 1.

285 286

Normalized weight (g) Substance

Cathode: Air

Anode: Zn

Weight (g)

(for 1kg battery)

(for 1kWh energy stored)

Carbon black

0.028

15.42

46.46

MnO2

0.010

5.51

16.59

PVDF

0.002

1.10

3.32

PTFE

0.015

8.56

25.80

PP

0.006

3.13

9.42

Nickel mesh

0.056

31.00

93.41

THF

0.500

276.55

833.33

Zn powder

0.500

276.55

833.33

Polyvinyl alcohol

0.196

108.16

325.93

Polyvinyl acetate

0.027

14.75

44.44

KOH

0.660

365.04

1100.00

H2O

0.309

170.78

514.63

H2O MilliporeTM

5.000

2765.49

8333.33

Membrane

Electrolyte

287

Table 1. Life Cycle Inventory of materials used to fabricate the Zn/Air battery using the process

288

described in the text. The weight of materials and solvents used for the prototype fabrication

289

(1.808 g) is provided in the third column and extrapolated to 1 kg battery as FU and 1kWh

290

stored energy in the fourth and fifth column respectively (considering 331.85Wh/kg capacity).

13

291

The electricity consumption during laboratory fabrication of the battery is summarized

292

in Table S3, where “use factor” of 0.7 has been applied. This use factor can be

293

improved in an industrial up-scaling of the process, but in this article a small use factor

294

has been considered in order to provide a cap for CED calculation and associated

295

emissions. Further reductions to the overall power demand could be envisaged

296

(sometimes reducing one hundred times of laboratory single sample process when

297

compared to industrial processing(Zackrisson et al., 2016). Other energy inputs of

298

subassemblies include energy inputs in raw material extraction and processing.

299

Considering all contributions, the total required energy in all materials and assembly

300

processes is 591 MJ per kg of battery, as shown in Figure 2 (left), and considering a

301

capacity for primary battery of 331.85Wh/kg (for one single cycle as measured in the

302

laboratory), this is equivalent to 1780.3MJ per 1kWh of stored energy, being the relative

303

share of each subassembly to the total CED equal in both cases. This value is higher

304

than others previously reported for Li-ion battery which are around 180 MJ per kg of

305

battery for industrial processes(McManus, 2012; Sullivan and Gaines, 2012). The

306

calculated value is higher because the fabrication route at laboratory scale is not

307

optimised, but since the main contribution is electricity consumption, a long way ahead

308

for improvement is expected in a scaled-up process and therefore, Zn-air battery

309

production at industrial scale could compete with lower CED values. Additionally, if

310

energy consumption during recycling processes is taken into account, an additional 20-

311

25 MJ per 1 kg of battery of CED should be added, estimated with data from reference

312

(Spanos et al., 2015). The embedded emissions for all materials and processes is 20.3 kg

313

CO2-eq per 1kg of battery (61.2 kg CO2-eq per 1kWh of stored energy), and the

314

contribution of all assemblies is presented in Figure 2 (right). Conventional Li-ion

14

315

batteries have values of 14.19 kg CO2-eq per 1kg or more recently reported impacts for

316

redox flow batteries, of which the all-vanadium type has lower embedded emissions

317

(2.86 kg CO2-eq per 1kg), both have lower values (even if transport has been included

318

in LCA), therefore pointing to all vanadium redox-flow as the battery with lower

319

emission impacts(Fernandez-Marchante et al., 2019).

320

321 322

Figure 2. Cumulative Energy Demand (CED, 590.8MJ per kg, or 1780.3MJ per 1kWh stored

323

energy) and embedded emissions (20.3 kg CO2-eq per kg, or 61.2 per 1kWh stored enegy) in all

324

materials and share for sub-assembly processess for Zn/air battery, where numbers are provided

325

for 1kg of battery, but % is valid for both cases (1kg of battery or 1kWh of stored energy).

326 327

The impacts in fourteen categories have been calculated according to ILCD

328

methodology; the results are summarized in Figure 3.A and B, where four main

329

categories present much higher contribution to total impact: Human Toxicity (cancer

330

and non-cancer effects), Freshwater Ecotoxicity and Resource Depletion. The other

331

categories have much lower contributions; note that in the figure 3.A and 3.B have

332

different mPt scale. These are similar results to other battery technologies, such as Li-

333

ion or all VRFB, where these three categories are also the main contributors to global

334

impacts, where clearly the lower impacts are for VRFB (four times lower)(Fernandez-

15

335

Marchante et al., 2019). The share of each subassembly to the fourteen categories is

336

presented in Figure 3C.

337

Figure 3. The impact of Zn/Air battery fabrication in fourteen categories (ILCD methodology), A) the most important impacts and B), the remaining ones Note the different mPt scale in the Y-axis. C) Share of the different Zn/Air battery subassemblies to the fourteen impact categories analysed (ILCD methodology). 338 339

Also, once the different processes during the battery assembly have been analysed, the

340

contribution of each of the processes to total impact have been evaluated and are

341

presented in Table S4.

16

342

From this single score analysis, it is clear that the electrolyte creates the lower impact

343

(less than 1% in both cases), while Zn anode has an important impact on resource

344

depletion. Globally, the cathode production presents the highest impacts, followed by

345

membrane production; both results are affected by the inclusion of long-term emissions,

346

which significantly increase the impact of both components.

347

The process contribution to impacts in the four main categories are presented in Table

348

S5 using ILCD methodology; this analysis detects the processes that are having higher

349

impacts on Human Toxicity (non-cancer and cancer), Freshwater, Ecotoxicity and

350

Resources Depletion, and therefore points to the steps for industrial up-scaling that

351

deserve more attention to be improved for a more sustainable production.

352

The contribution of each subassembly to all the categories are presented as stacked

353

columns in Figure 4 (absolute mPt ILCD scale), this graph emphasizes the contribution

354

to the main categories, specially Zn to resource depletion category, accounting for more

355

than 50% of global score. If this mineral depletion is not considered, cathode is then the

356

principal contributor, followed by membrane, both have the strongest impact on

357

Freshwater ecotoxicity and Human toxicity.

17

358 359

Figure 4. Stacked impact contribution of each subassembly of the Zn/Air battery to the fourteen

360

impact categories analysed by ILCD methodology.

361 362

All impact categories can be organized in three main groups, which deliver the

363

following quantified impacts, presented in this case by using ReCiPe methodology for

364

comparison: Human health, Ecosystems and Resources depletion grouping has been

365

applied. The relative contribution of each battery subassembly is presented in Figure S3

366

for this method, which provides a lower impact of Zn on resource depletion than ILCD,

367

but still keeping similar results in other categories.

368

Finally, an estimation of the economic cost of energy service by the Zn/Air battery has

369

been carried out, this is a preliminary result, since the assumption of a number of cycles

370

is based on promising progress for secondary Zn/Air battery already commented in the

371

introduction section, but not on the battery synthesized in the laboratory. In Figure 5A a

372

comparison of capital cost for power and energy in different battery technologies are

373

presented. For 1500 cycles the power cost of Zn/air battery is 2,3 k$/kW, the lowest of

374

current technologies. Besides, the Zn/Air battery already has the lower capital cost and

375

could become more competitive if cyclability is improved: a calculation for 2000 cycles

18

376

delivers a capital cost for energy storage around 100 $/MWh/cycles, better than most of

377

current technologies.

378

The cost of the energy service is strongly dependant on the final number of cycles that

379

the battery could provide (including initial cycle efficiency and a loss of performance

380

with increasing number of cycles); therefore, and taking into account an initial cost of

381

the battery of 200 USD/kWh, a sensitivity analysis is presented in Figure 5B, where the

382

energy service cost per MWh is presented depending on the number of cycles and the

383

efficiency per cycle, including one plot with a 0.0025% loss per cycle.

384

19

385 386

Figure 5.A) Comparison of capital cost for power (blue, left axis) and energy (orange, right

387

axis) for several current technologies (References: a:(Alotto et al., 2014), b: Vanadium redox-

388

flow and others (Dehghani-Sanij et al., 2019; Zakeri and Syri, 2015), c:(Posada et al., 2017))

389

and the calculation for the maximum limits for Zn/Air battery taking into account a cyclability

390

of 1500 cycles. B) Levelized cost of the stored energy throughout lifetime as a function of the

20

391

number of cycles of the battery (up to 2000 cycles), depending on the efficiency and including a

392

case with cycle losses (broken line).

393

4. Conclusions

394

A detailed LCA of the fabrication process of a Zn/Air battery has been carried out using

395

ILCD methodology (some results are also confirmed by ReCiPe methodology). In the

396

analysis, extraction and processing of raw materials and assembly of different battery

397

components have been considered. Also, a cumulative energy demand (CED) analysis

398

has been carried out, indicating that the Zn/Air battery fabrication has a CED of 590.8

399

MJ per 1kg of fabricated battery (1780.3MJ per 1kWh of stored energy). This value is

400

higher than others published for Li-ion batteries or redox-flow batteries, which is due to

401

the laboratory scale of the production (a moderate use factor of equipment of 0.7 has

402

been considered), there is room for improvement if an industrial scale fabrication is

403

accomplished.

404

This CED generates emissions amounting to 20.3kg CO2-eq per kg of fabricated Zn/Air

405

battery (61.2 kg CO2-eq per 1kWh of stored energy). When the impacts in different

406

categories are analyzed, Resource Depletion (due mostly to Zn consumption) followed

407

by Human Toxicity and Freshwater ecotoxicity have the highest score; similar to other

408

battery technologies, while other categories have lower impacts. When analyzed in

409

detail, the fabrication of cathode is the subassembly process that generates the highest

410

impacts, followed by the membrane, and therefore a recommendation is to focus on the

411

optimization of these subassemblies fabrication process in an up-scaled industrial

412

manufacture. It has to be noted that the aim of this article is to stablish a reference of a

413

Zn/air battery synthesized in the laboratory, providing support for metal/air battery

414

developers about environmental hotspots and allowing to be used by other researchers

21

415

for comparison with a base value of several categories of impact. It is clear that the

416

optimization of the process in an up-scaled industrial manufacture will improve

417

substantially the CED, emissions, impacts and cost values. In this sense, as an example,

418

simply considering that five membranes and more than 100 cathode electrodes can be

419

synthesized using the same hot plate and laboratory oven respectively, the energy

420

consumption decreases from 246.7 MJ/kg to 19.9 MJ/kg.

421

Finally, a preliminary economic cost assessment has been carried out, in a horizon of

422

cyclability in the range of 500 to 2000 cycles, the energy stored in the battery can be

423

provided as an electricity service at a competitive cost when compared with other

424

battery technologies already in the market.

425 426

427

Acknowledgments

428

This work was supported by: Ministerio de Ciencia, Innovación y Universidades

429

AEI/FEDER/UE (Spain, Refs: ENE2016-79282-C5-5-R, CTQ2017-90659-REDT and

430

MAT2015-65274-R) and Fundación Séneca (Spain, Ref:19882-GERM-15 and

431

Ref:20985/PI/18), both including EU Feder funds. C.T. is grateful to F. Séneca for PhD

432

grant (Exp. 19768/FPI/15).

433

434

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Highlights •

A cradle-to-gate LCA of a laboratory-synthesized Zn/air battery has been carried out

•

The power cost of Zn/air batteries is the lowest of current technologies

•

The cathode production presents the highest environmental impacts

•

Zn/Air battery should be competitive if cyclability is moderately improved

•

A capital cost for energy storage around 100 $/kWh/cycles was obtained

Author Contributions F. Santos*. Investigation, Writing-Reviewing & editing, Visualization A. Urbina*: Conceptualization, Data Curation, Supervision, Writing-Original Draft, Methodology. J. Abad: Investigation, Writing-Reviewing & editing R. López: Data Curation, Visualization C. Toledo: Data Curation, Formal Analysis A.J. Fernández Romero: Investigation, Supervision, Writing-review & editing, Visualization

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: